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Benefit Plans 2 (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2018
Dec. 31, 2017
Pension Plan [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class $ 4,827.3 $ 4,695.9 $ 5,174.1
Year End Allocation      
Total   100.00% 100.00%
Reconciliation of Changes in Fair Value of Assets Classified as Level 3 in Fair Value Hierarchy for the Pension Assets      
Fair Value of Plan Assets as of January 1 4,827.3    
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 5,174.1    
Pension Plan [Member] | AEP Texas Inc. [Member]      
Allocated Assets of Investments   8.70% 8.80%
Pension and Other Postretirement Plans' Assets      
Asset Class 416.6 $ 410.7 $ 455.9
Reconciliation of Changes in Fair Value of Assets Classified as Level 3 in Fair Value Hierarchy for the Pension Assets      
Fair Value of Plan Assets as of January 1 416.6    
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 455.9    
Pension Plan [Member] | Appalachian Power Co [Member]      
Allocated Assets of Investments   12.60% 12.60%
Pension and Other Postretirement Plans' Assets      
Asset Class 606.4 $ 593.3 $ 651.7
Reconciliation of Changes in Fair Value of Assets Classified as Level 3 in Fair Value Hierarchy for the Pension Assets      
Fair Value of Plan Assets as of January 1 606.4    
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 651.7    
Pension Plan [Member] | Indiana Michigan Power Co [Member]      
Allocated Assets of Investments   12.40% 12.30%
Pension and Other Postretirement Plans' Assets      
Asset Class 586.1 $ 583.8 $ 636.7
Reconciliation of Changes in Fair Value of Assets Classified as Level 3 in Fair Value Hierarchy for the Pension Assets      
Fair Value of Plan Assets as of January 1 586.1    
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 636.7    
Pension Plan [Member] | Ohio Power Co [Member]      
Allocated Assets of Investments   9.90% 9.80%
Pension and Other Postretirement Plans' Assets      
Asset Class 473.8 $ 466.1 $ 509.1
Reconciliation of Changes in Fair Value of Assets Classified as Level 3 in Fair Value Hierarchy for the Pension Assets      
Fair Value of Plan Assets as of January 1 473.8    
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 509.1    
Pension Plan [Member] | Public Service Co Of Oklahoma [Member]      
Allocated Assets of Investments   5.60% 5.60%
Pension and Other Postretirement Plans' Assets      
Asset Class 266.0 $ 261.2 $ 287.8
Reconciliation of Changes in Fair Value of Assets Classified as Level 3 in Fair Value Hierarchy for the Pension Assets      
Fair Value of Plan Assets as of January 1 266.0    
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 287.8    
Pension Plan [Member] | Southwestern Electric Power Co [Member]      
Allocated Assets of Investments   6.00% 6.00%
Pension and Other Postretirement Plans' Assets      
Asset Class 287.3 $ 281.0 $ 311.7
Reconciliation of Changes in Fair Value of Assets Classified as Level 3 in Fair Value Hierarchy for the Pension Assets      
Fair Value of Plan Assets as of January 1 287.3    
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 311.7    
Other Postretirement Benefit Plans [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class 1,545.9 $ 1,534.2 $ 1,732.5
Year End Allocation      
Total   100.00% 100.00%
Reconciliation of Changes in Fair Value of Assets Classified as Level 3 in Fair Value Hierarchy for the Pension Assets      
Fair Value of Plan Assets as of January 1 1,545.9    
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 1,732.5    
Other Postretirement Benefit Plans [Member] | AEP Texas Inc. [Member]      
Allocated Assets of Investments   8.50% 8.50%
Pension and Other Postretirement Plans' Assets      
Asset Class 134.1 $ 129.9 $ 147.3
Reconciliation of Changes in Fair Value of Assets Classified as Level 3 in Fair Value Hierarchy for the Pension Assets      
Fair Value of Plan Assets as of January 1 134.1    
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 147.3    
Other Postretirement Benefit Plans [Member] | Appalachian Power Co [Member]      
Allocated Assets of Investments   15.50% 15.80%
Pension and Other Postretirement Plans' Assets      
Asset Class 246.9 $ 238.4 $ 273.4
Reconciliation of Changes in Fair Value of Assets Classified as Level 3 in Fair Value Hierarchy for the Pension Assets      
Fair Value of Plan Assets as of January 1 246.9    
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 273.4    
Other Postretirement Benefit Plans [Member] | Indiana Michigan Power Co [Member]      
Allocated Assets of Investments   12.20% 12.20%
Pension and Other Postretirement Plans' Assets      
Asset Class 186.6 $ 187.3 $ 211.1
Reconciliation of Changes in Fair Value of Assets Classified as Level 3 in Fair Value Hierarchy for the Pension Assets      
Fair Value of Plan Assets as of January 1 186.6    
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 211.1    
Other Postretirement Benefit Plans [Member] | Ohio Power Co [Member]      
Allocated Assets of Investments   11.40% 11.50%
Pension and Other Postretirement Plans' Assets      
Asset Class 182.6 $ 175.4 $ 198.5
Reconciliation of Changes in Fair Value of Assets Classified as Level 3 in Fair Value Hierarchy for the Pension Assets      
Fair Value of Plan Assets as of January 1 182.6    
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 198.5    
Other Postretirement Benefit Plans [Member] | Public Service Co Of Oklahoma [Member]      
Allocated Assets of Investments   5.50% 5.50%
Pension and Other Postretirement Plans' Assets      
Asset Class 86.4 $ 84.3 $ 95.5
Reconciliation of Changes in Fair Value of Assets Classified as Level 3 in Fair Value Hierarchy for the Pension Assets      
Fair Value of Plan Assets as of January 1 86.4    
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 95.5    
Other Postretirement Benefit Plans [Member] | Southwestern Electric Power Co [Member]      
Allocated Assets of Investments   6.40% 6.40%
Pension and Other Postretirement Plans' Assets      
Asset Class 96.8 $ 98.5 $ 110.4
Reconciliation of Changes in Fair Value of Assets Classified as Level 3 in Fair Value Hierarchy for the Pension Assets      
Fair Value of Plan Assets as of January 1 96.8    
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 110.4    
Other [Member] | Pension Plan [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class 1,292.6 1,127.5 1,292.6
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 1,292.6    
Other [Member] | Other Postretirement Benefit Plans [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class 339.9 393.4 339.9
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 339.9    
Level 1 [Member] | Pension Plan [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class 826.7 661.2 826.7
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 826.7    
Level 1 [Member] | Other Postretirement Benefit Plans [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class 700.4 515.6 700.4
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 700.4    
Level 2 [Member] | Pension Plan [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class 3,054.8 2,907.2 3,054.8
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 3,054.8    
Level 2 [Member] | Other Postretirement Benefit Plans [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class 692.2 625.2 692.2
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 692.2    
Level 3 [Member] | Pension Plan [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class 723.6 0.0 0.0
Reconciliation of Changes in Fair Value of Assets Classified as Level 3 in Fair Value Hierarchy for the Pension Assets      
Fair Value of Plan Assets as of January 1 723.6    
Actual Return on Plan Assets      
Relating to Assets Still Held as of the Reporting Date 0.0    
Relating to Assets Sold During the Period 0.0    
Purchases and Sales 0.0    
Transfers into Level 3 0.0    
Transfers out of Level 3 [1] (723.6)    
Fair Value of Plan Assets as of December 31 0.0    
Level 3 [Member] | Other Postretirement Benefit Plans [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class 0.0 0.0 0.0
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 0.0    
Equity Securities [Member] | Pension Plan [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class [2] 1,306.1 $ 1,049.8 $ 1,306.1
Year End Allocation      
Total [2]   22.40% 25.20%
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 [2] 1,306.1    
Equity Securities [Member] | Other Postretirement Benefit Plans [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class 777.0 $ 649.7 $ 777.0
Year End Allocation      
Total   42.30% 44.80%
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 777.0    
Equity Securities [Member] | Other [Member] | Pension Plan [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class [2] 452.9 $ 370.1 $ 452.9
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 [2] 452.9    
Equity Securities [Member] | Other [Member] | Other Postretirement Benefit Plans [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class 153.6 226.2 153.6
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 153.6    
Equity Securities [Member] | Level 1 [Member] | Pension Plan [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class [2] 826.3 661.4 826.3
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 [2] 826.3    
Equity Securities [Member] | Level 1 [Member] | Other Postretirement Benefit Plans [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class 614.0 419.2 614.0
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 614.0    
Equity Securities [Member] | Level 2 [Member] | Pension Plan [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class [2] 26.9 18.3 26.9
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 [2] 26.9    
Equity Securities [Member] | Level 2 [Member] | Other Postretirement Benefit Plans [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class 9.4 4.3 9.4
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 9.4    
Equity Securities [Member] | Level 3 [Member] | Pension Plan [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class [2] 0.0 0.0 0.0
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 [2] 0.0    
Equity Securities [Member] | Level 3 [Member] | Other Postretirement Benefit Plans [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class 0.0 0.0 0.0
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 0.0    
Domestic [Member] | Pension Plan [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class [2] 318.6 $ 277.3 $ 318.6
Year End Allocation      
Total [2]   5.90% 6.20%
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 [2] 318.6    
Domestic [Member] | Other Postretirement Benefit Plans [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class 307.1 $ 233.3 $ 307.1
Year End Allocation      
Total   15.20% 17.70%
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 307.1    
Domestic [Member] | Other [Member] | Pension Plan [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class [2] 0.0 $ 0.0 $ 0.0
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 [2] 0.0    
Domestic [Member] | Other [Member] | Other Postretirement Benefit Plans [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class 0.0 0.0 0.0
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 0.0    
Domestic [Member] | Level 1 [Member] | Pension Plan [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class [2] 318.6 277.3 318.6
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 [2] 318.6    
Domestic [Member] | Level 1 [Member] | Other Postretirement Benefit Plans [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class 307.1 233.3 307.1
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 307.1    
Domestic [Member] | Level 2 [Member] | Pension Plan [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class [2] 0.0 0.0 0.0
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 [2] 0.0    
Domestic [Member] | Level 2 [Member] | Other Postretirement Benefit Plans [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class 0.0 0.0 0.0
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 0.0    
Domestic [Member] | Level 3 [Member] | Pension Plan [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class [2] 0.0 0.0 0.0
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 [2] 0.0    
Domestic [Member] | Level 3 [Member] | Other Postretirement Benefit Plans [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class 0.0 0.0 0.0
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 0.0    
International [Member] | Pension Plan [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class [2] 507.7 $ 384.1 $ 507.7
Year End Allocation      
Total [2]   8.20% 9.80%
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 [2] 507.7    
International [Member] | Other Postretirement Benefit Plans [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class 306.9 $ 185.9 $ 306.9
Year End Allocation      
Total   12.10% 17.70%
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 306.9    
International [Member] | Other [Member] | Pension Plan [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class [2] 0.0 $ 0.0 $ 0.0
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 [2] 0.0    
International [Member] | Other [Member] | Other Postretirement Benefit Plans [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class 0.0 0.0 0.0
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 0.0    
International [Member] | Level 1 [Member] | Pension Plan [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class [2] 507.7 384.1 507.7
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 [2] 507.7    
International [Member] | Level 1 [Member] | Other Postretirement Benefit Plans [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class 306.9 185.9 306.9
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 306.9    
International [Member] | Level 2 [Member] | Pension Plan [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class [2] 0.0 0.0 0.0
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 [2] 0.0    
International [Member] | Level 2 [Member] | Other Postretirement Benefit Plans [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class 0.0 0.0 0.0
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 0.0    
International [Member] | Level 3 [Member] | Pension Plan [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class [2] 0.0 0.0 0.0
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 [2] 0.0    
International [Member] | Level 3 [Member] | Other Postretirement Benefit Plans [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class 0.0 0.0 0.0
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 0.0    
Options [Member] | Pension Plan [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class [2] 26.9 $ 18.3 $ 26.9
Year End Allocation      
Total [2]   0.40% 0.50%
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 [2] 26.9    
Options [Member] | Other Postretirement Benefit Plans [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class 9.4 $ 4.3 $ 9.4
Year End Allocation      
Total   0.30% 0.50%
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 9.4    
Options [Member] | Other [Member] | Pension Plan [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class [2] 0.0 $ 0.0 $ 0.0
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 [2] 0.0    
Options [Member] | Other [Member] | Other Postretirement Benefit Plans [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class 0.0 0.0 0.0
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 0.0    
Options [Member] | Level 1 [Member] | Pension Plan [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class [2] 0.0 0.0 0.0
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 [2] 0.0    
Options [Member] | Level 1 [Member] | Other Postretirement Benefit Plans [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class 0.0 0.0 0.0
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 0.0    
Options [Member] | Level 2 [Member] | Pension Plan [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class [2] 26.9 18.3 26.9
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 [2] 26.9    
Options [Member] | Level 2 [Member] | Other Postretirement Benefit Plans [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class 9.4 4.3 9.4
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 9.4    
Options [Member] | Level 3 [Member] | Pension Plan [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class [2] 0.0 0.0 0.0
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 [2] 0.0    
Options [Member] | Level 3 [Member] | Other Postretirement Benefit Plans [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class 0.0 0.0 0.0
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 0.0    
Common Collective Trusts [Member] | Pension Plan [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class [2],[3] 452.9 $ 370.1 $ 452.9
Year End Allocation      
Total [2],[3]   7.90% 8.70%
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 [2],[3] 452.9    
Common Collective Trusts [Member] | Other Postretirement Benefit Plans [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class [4] 153.6 $ 226.2 $ 153.6
Year End Allocation      
Total [4]   14.70% 8.90%
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 [4] 153.6    
Common Collective Trusts [Member] | Other [Member] | Pension Plan [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class [2],[3] 452.9 $ 370.1 $ 452.9
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 [2],[3] 452.9    
Common Collective Trusts [Member] | Other [Member] | Other Postretirement Benefit Plans [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class [4] 153.6 226.2 153.6
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 [4] 153.6    
Common Collective Trusts [Member] | Level 1 [Member] | Pension Plan [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class [2],[3] 0.0 0.0 0.0
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 [2],[3] 0.0    
Common Collective Trusts [Member] | Level 1 [Member] | Other Postretirement Benefit Plans [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class [4] 0.0 0.0 0.0
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 [4] 0.0    
Common Collective Trusts [Member] | Level 2 [Member] | Pension Plan [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class [2],[3] 0.0 0.0 0.0
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 [2],[3] 0.0    
Common Collective Trusts [Member] | Level 2 [Member] | Other Postretirement Benefit Plans [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class [4] 0.0 0.0 0.0
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 [4] 0.0    
Common Collective Trusts [Member] | Level 3 [Member] | Pension Plan [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class [2],[3] 0.0 0.0 0.0
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 [2],[3] 0.0    
Common Collective Trusts [Member] | Level 3 [Member] | Other Postretirement Benefit Plans [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class [4] 0.0 0.0 0.0
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 [4] 0.0    
Fixed Income [Member] | Pension Plan [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class [2] 2,992.3 $ 2,852.8 $ 2,992.3
Year End Allocation      
Total [2]   60.70% 57.80%
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 [2] 2,992.3    
Fixed Income [Member] | Other Postretirement Benefit Plans [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class 693.9 $ 622.7 $ 693.9
Year End Allocation      
Total   40.60% 40.10%
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 693.9    
Fixed Income [Member] | Other [Member] | Pension Plan [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class [2] 0.0 $ 0.0 $ 0.0
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 [2] 0.0    
Fixed Income [Member] | Other [Member] | Other Postretirement Benefit Plans [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class 185.0 163.6 185.0
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 185.0    
Fixed Income [Member] | Level 1 [Member] | Pension Plan [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class [2] 0.0 0.2 0.0
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 [2] 0.0    
Fixed Income [Member] | Level 1 [Member] | Other Postretirement Benefit Plans [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class 49.7 42.0 49.7
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 49.7    
Fixed Income [Member] | Level 2 [Member] | Pension Plan [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class [2] 2,992.3 2,852.6 2,992.3
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 [2] 2,992.3    
Fixed Income [Member] | Level 2 [Member] | Other Postretirement Benefit Plans [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class 459.2 417.1 459.2
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 459.2    
Fixed Income [Member] | Level 3 [Member] | Pension Plan [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class [2] 0.0 0.0 0.0
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 [2] 0.0    
Fixed Income [Member] | Level 3 [Member] | Other Postretirement Benefit Plans [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class 0.0 0.0 0.0
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 0.0    
Common Collective Trust - Debt [Member] | Other Postretirement Benefit Plans [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class [4] 185.0 $ 163.6 $ 185.0
Year End Allocation      
Total [4]   10.70% 10.70%
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 [4] 185.0    
Common Collective Trust - Debt [Member] | Other [Member] | Other Postretirement Benefit Plans [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class [4] 185.0 $ 163.6 $ 185.0
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 [4] 185.0    
Common Collective Trust - Debt [Member] | Level 1 [Member] | Other Postretirement Benefit Plans [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class [4] 0.0 0.0 0.0
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 [4] 0.0    
Common Collective Trust - Debt [Member] | Level 2 [Member] | Other Postretirement Benefit Plans [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class [4] 0.0 0.0 0.0
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 [4] 0.0    
Common Collective Trust - Debt [Member] | Level 3 [Member] | Other Postretirement Benefit Plans [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class [4] 0.0 0.0 0.0
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 [4] 0.0    
United States Government and Agency Securities [Member] | Pension Plan [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class [2] 1,376.5 $ 1,512.7 $ 1,376.5
Year End Allocation      
Total [2]   32.20% 26.60%
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 [2] 1,376.5    
United States Government and Agency Securities [Member] | Other Postretirement Benefit Plans [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class 187.4 $ 181.7 $ 187.4
Year End Allocation      
Total   11.80% 10.80%
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 187.4    
United States Government and Agency Securities [Member] | Other [Member] | Pension Plan [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class [2] 0.0 $ 0.0 $ 0.0
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 [2] 0.0    
United States Government and Agency Securities [Member] | Other [Member] | Other Postretirement Benefit Plans [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class 0.0 0.0 0.0
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 0.0    
United States Government and Agency Securities [Member] | Level 1 [Member] | Pension Plan [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class [2] 0.0 0.2 0.0
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 [2] 0.0    
United States Government and Agency Securities [Member] | Level 1 [Member] | Other Postretirement Benefit Plans [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class 0.0 0.2 0.0
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 0.0    
United States Government and Agency Securities [Member] | Level 2 [Member] | Pension Plan [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class [2] 1,376.5 1,512.5 1,376.5
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 [2] 1,376.5    
United States Government and Agency Securities [Member] | Level 2 [Member] | Other Postretirement Benefit Plans [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class 187.4 181.5 187.4
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 187.4    
United States Government and Agency Securities [Member] | Level 3 [Member] | Pension Plan [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class [2] 0.0 0.0 0.0
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 [2] 0.0    
United States Government and Agency Securities [Member] | Level 3 [Member] | Other Postretirement Benefit Plans [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class 0.0 0.0 0.0
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 0.0    
Corporate Debt [Member] | Pension Plan [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class [2] 1,277.0 $ 1,082.9 $ 1,277.0
Year End Allocation      
Total [2]   23.00% 24.70%
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 [2] 1,277.0    
Corporate Debt [Member] | Other Postretirement Benefit Plans [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class 214.1 $ 188.6 $ 214.1
Year End Allocation      
Total   12.30% 12.40%
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 214.1    
Corporate Debt [Member] | Other [Member] | Pension Plan [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class [2] 0.0 $ 0.0 $ 0.0
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 [2] 0.0    
Corporate Debt [Member] | Other [Member] | Other Postretirement Benefit Plans [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class 0.0 0.0 0.0
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 0.0    
Corporate Debt [Member] | Level 1 [Member] | Pension Plan [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class [2] 0.0 0.0 0.0
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 [2] 0.0    
Corporate Debt [Member] | Level 1 [Member] | Other Postretirement Benefit Plans [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class 0.0 0.0 0.0
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 0.0    
Corporate Debt [Member] | Level 2 [Member] | Pension Plan [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class [2] 1,277.0 1,082.9 1,277.0
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 [2] 1,277.0    
Corporate Debt [Member] | Level 2 [Member] | Other Postretirement Benefit Plans [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class 214.1 188.6 214.1
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 214.1    
Corporate Debt [Member] | Level 3 [Member] | Pension Plan [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class [2] 0.0 0.0 0.0
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 [2] 0.0    
Corporate Debt [Member] | Level 3 [Member] | Other Postretirement Benefit Plans [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class 0.0 0.0 0.0
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 0.0    
Foreign Debt [Member] | Pension Plan [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class [2] 296.9 $ 221.6 $ 296.9
Year End Allocation      
Total [2]   4.70% 5.70%
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 [2] 296.9    
Foreign Debt [Member] | Other Postretirement Benefit Plans [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class 40.7 $ 35.0 $ 40.7
Year End Allocation      
Total   2.30% 2.40%
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 40.7    
Foreign Debt [Member] | Other [Member] | Pension Plan [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class [2] 0.0 $ 0.0 $ 0.0
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 [2] 0.0    
Foreign Debt [Member] | Other [Member] | Other Postretirement Benefit Plans [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class 0.0 0.0 0.0
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 0.0    
Foreign Debt [Member] | Level 1 [Member] | Pension Plan [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class [2] 0.0 0.0 0.0
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 [2] 0.0    
Foreign Debt [Member] | Level 1 [Member] | Other Postretirement Benefit Plans [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class 0.0 0.0 0.0
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 0.0    
Foreign Debt [Member] | Level 2 [Member] | Pension Plan [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class [2] 296.9 221.6 296.9
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 [2] 296.9    
Foreign Debt [Member] | Level 2 [Member] | Other Postretirement Benefit Plans [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class 40.7 35.0 40.7
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 40.7    
Foreign Debt [Member] | Level 3 [Member] | Pension Plan [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class [2] 0.0 0.0 0.0
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 [2] 0.0    
Foreign Debt [Member] | Level 3 [Member] | Other Postretirement Benefit Plans [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class 0.0 0.0 0.0
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 0.0    
State and Local Government [Member] | Pension Plan [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class [2] 31.7 $ 28.2 $ 31.7
Year End Allocation      
Total [2]   0.60% 0.60%
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 [2] 31.7    
State and Local Government [Member] | Other Postretirement Benefit Plans [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class 66.5 $ 53.6 $ 66.5
Year End Allocation      
Total   3.50% 3.80%
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 66.5    
State and Local Government [Member] | Other [Member] | Pension Plan [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class [2] 0.0 $ 0.0 $ 0.0
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 [2] 0.0    
State and Local Government [Member] | Other [Member] | Other Postretirement Benefit Plans [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class 0.0 0.0 0.0
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 0.0    
State and Local Government [Member] | Level 1 [Member] | Pension Plan [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class [2] 0.0 0.0 0.0
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 [2] 0.0    
State and Local Government [Member] | Level 1 [Member] | Other Postretirement Benefit Plans [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class 49.7 41.8 49.7
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 49.7    
State and Local Government [Member] | Level 2 [Member] | Pension Plan [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class [2] 31.7 28.2 31.7
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 [2] 31.7    
State and Local Government [Member] | Level 2 [Member] | Other Postretirement Benefit Plans [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class 16.8 11.8 16.8
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 16.8    
State and Local Government [Member] | Level 3 [Member] | Pension Plan [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class [2] 0.0 0.0 0.0
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 [2] 0.0    
State and Local Government [Member] | Level 3 [Member] | Other Postretirement Benefit Plans [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class 0.0 0.0 0.0
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 0.0    
Other - Asset Backed [Member] | Pension Plan [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class [2] 10.2 $ 7.4 $ 10.2
Year End Allocation      
Total [2]   0.20% 0.20%
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 [2] 10.2    
Other - Asset Backed [Member] | Other Postretirement Benefit Plans [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class 0.2 $ 0.2 $ 0.2
Year End Allocation      
Total   0.00% 0.00%
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 0.2    
Other - Asset Backed [Member] | Other [Member] | Pension Plan [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class [2] 0.0 $ 0.0 $ 0.0
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 [2] 0.0    
Other - Asset Backed [Member] | Other [Member] | Other Postretirement Benefit Plans [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class 0.0 0.0 0.0
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 0.0    
Other - Asset Backed [Member] | Level 1 [Member] | Pension Plan [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class [2] 0.0 0.0 0.0
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 [2] 0.0    
Other - Asset Backed [Member] | Level 1 [Member] | Other Postretirement Benefit Plans [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class 0.0 0.0 0.0
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 0.0    
Other - Asset Backed [Member] | Level 2 [Member] | Pension Plan [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class [2] 10.2 7.4 10.2
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 [2] 10.2    
Other - Asset Backed [Member] | Level 2 [Member] | Other Postretirement Benefit Plans [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class 0.2 0.2 0.2
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 0.2    
Other - Asset Backed [Member] | Level 3 [Member] | Pension Plan [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class [2] 0.0 0.0 0.0
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 [2] 0.0    
Other - Asset Backed [Member] | Level 3 [Member] | Other Postretirement Benefit Plans [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class 0.0 0.0 0.0
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 0.0    
Infrastructure [Member] | Pension Plan [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class [3] 59.5 $ 72.2 $ 59.5
Year End Allocation      
Total [3]   1.50% 1.20%
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 [3] 59.5    
Infrastructure [Member] | Other [Member] | Pension Plan [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class [3] 59.5 $ 72.2 $ 59.5
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 [3] 59.5    
Infrastructure [Member] | Level 1 [Member] | Pension Plan [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class [3] 0.0 0.0 0.0
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 [3] 0.0    
Infrastructure [Member] | Level 2 [Member] | Pension Plan [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class [3] 0.0 0.0 0.0
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 [3] 0.0    
Infrastructure [Member] | Level 3 [Member] | Pension Plan [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class 57.6 0.0 [3] 0.0 [3]
Reconciliation of Changes in Fair Value of Assets Classified as Level 3 in Fair Value Hierarchy for the Pension Assets      
Fair Value of Plan Assets as of January 1 57.6    
Actual Return on Plan Assets      
Relating to Assets Still Held as of the Reporting Date 0.0    
Relating to Assets Sold During the Period 0.0    
Purchases and Sales 0.0    
Transfers into Level 3 0.0    
Transfers out of Level 3 [1] (57.6)    
Fair Value of Plan Assets as of December 31 [3] 0.0    
Real Estate [Member] | Pension Plan [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class [3] 290.3 $ 220.4 $ 290.3
Year End Allocation      
Total [3]   4.70% 5.60%
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 [3] 290.3    
Real Estate [Member] | Other [Member] | Pension Plan [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class [3] 290.3 $ 220.4 $ 290.3
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 [3] 290.3    
Real Estate [Member] | Level 1 [Member] | Pension Plan [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class [3] 0.0 0.0 0.0
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 [3] 0.0    
Real Estate [Member] | Level 2 [Member] | Pension Plan [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class [3] 0.0 0.0 0.0
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 [3] 0.0    
Real Estate [Member] | Level 3 [Member] | Pension Plan [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class 254.9 0.0 [3] 0.0 [3]
Reconciliation of Changes in Fair Value of Assets Classified as Level 3 in Fair Value Hierarchy for the Pension Assets      
Fair Value of Plan Assets as of January 1 254.9    
Actual Return on Plan Assets      
Relating to Assets Still Held as of the Reporting Date 0.0    
Relating to Assets Sold During the Period 0.0    
Purchases and Sales 0.0    
Transfers into Level 3 0.0    
Transfers out of Level 3 [1] (254.9)    
Fair Value of Plan Assets as of December 31 [3] 0.0    
Alternative Investments [Member] | Pension Plan [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class [3] 446.0 $ 444.6 $ 446.0
Year End Allocation      
Total [3]   9.50% 8.60%
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 [3] 446.0    
Alternative Investments [Member] | Other [Member] | Pension Plan [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class [3] 446.0 $ 444.6 $ 446.0
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 [3] 446.0    
Alternative Investments [Member] | Level 1 [Member] | Pension Plan [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class [3] 0.0 0.0 0.0
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 [3] 0.0    
Alternative Investments [Member] | Level 2 [Member] | Pension Plan [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class [3] 0.0 0.0 0.0
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 [3] 0.0    
Alternative Investments [Member] | Level 3 [Member] | Pension Plan [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class 411.1 0.0 [3] 0.0 [3]
Reconciliation of Changes in Fair Value of Assets Classified as Level 3 in Fair Value Hierarchy for the Pension Assets      
Fair Value of Plan Assets as of January 1 411.1    
Actual Return on Plan Assets      
Relating to Assets Still Held as of the Reporting Date 0.0    
Relating to Assets Sold During the Period 0.0    
Purchases and Sales 0.0    
Transfers into Level 3 0.0    
Transfers out of Level 3 [1] (411.1)    
Fair Value of Plan Assets as of December 31 [3] 0.0    
Trusted Owned Life Insurance [Member] | Other Postretirement Benefit Plans [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class 223.6 $ 203.8 $ 223.6
Year End Allocation      
Total   13.30% 12.90%
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 223.6    
Trusted Owned Life Insurance [Member] | Other [Member] | Other Postretirement Benefit Plans [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class 0.0 $ 0.0 $ 0.0
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 0.0    
Trusted Owned Life Insurance [Member] | Level 1 [Member] | Other Postretirement Benefit Plans [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class 0.0 0.0 0.0
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 0.0    
Trusted Owned Life Insurance [Member] | Level 2 [Member] | Other Postretirement Benefit Plans [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class 223.6 203.8 223.6
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 223.6    
Trusted Owned Life Insurance [Member] | Level 3 [Member] | Other Postretirement Benefit Plans [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class 0.0 0.0 0.0
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 0.0    
International Equities [Member] | Other Postretirement Benefit Plans [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class 105.4 $ 49.4 $ 105.4
Year End Allocation      
Total   3.20% 6.10%
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 105.4    
International Equities [Member] | Other [Member] | Other Postretirement Benefit Plans [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class 0.0 $ 0.0 $ 0.0
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 0.0    
International Equities [Member] | Level 1 [Member] | Other Postretirement Benefit Plans [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class 0.0 0.0 0.0
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 0.0    
International Equities [Member] | Level 2 [Member] | Other Postretirement Benefit Plans [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class 105.4 49.4 105.4
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 105.4    
International Equities [Member] | Level 3 [Member] | Other Postretirement Benefit Plans [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class 0.0 0.0 0.0
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 0.0    
United States Bonds [Member] | Other Postretirement Benefit Plans [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class 118.2 $ 154.4 $ 118.2
Year End Allocation      
Total   10.10% 6.80%
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 118.2    
United States Bonds [Member] | Other [Member] | Other Postretirement Benefit Plans [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class 0.0 $ 0.0 $ 0.0
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 0.0    
United States Bonds [Member] | Level 1 [Member] | Other Postretirement Benefit Plans [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class 0.0 0.0 0.0
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 0.0    
United States Bonds [Member] | Level 2 [Member] | Other Postretirement Benefit Plans [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class 118.2 154.4 118.2
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 118.2    
United States Bonds [Member] | Level 3 [Member] | Other Postretirement Benefit Plans [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class 0.0 0.0 0.0
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 0.0    
Cash and Cash Equivalents [Member] | Pension Plan [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class [3] 57.2 $ 47.8 $ 57.2
Year End Allocation      
Total [3]   1.00% 1.10%
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 [3] 57.2    
Cash and Cash Equivalents [Member] | Other Postretirement Benefit Plans [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class [4] 40.9 $ 59.2 $ 40.9
Year End Allocation      
Total [4]   3.90% 2.40%
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 [4] 40.9    
Cash and Cash Equivalents [Member] | Other [Member] | Pension Plan [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class [3] 21.2 $ 11.9 $ 21.2
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 [3] 21.2    
Cash and Cash Equivalents [Member] | Other [Member] | Other Postretirement Benefit Plans [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class [4] 4.2 4.8 4.2
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 [4] 4.2    
Cash and Cash Equivalents [Member] | Level 1 [Member] | Pension Plan [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class [3] 0.4 (0.4) 0.4
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 [3] 0.4    
Cash and Cash Equivalents [Member] | Level 1 [Member] | Other Postretirement Benefit Plans [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class [4] 36.7 54.4 36.7
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 [4] 36.7    
Cash and Cash Equivalents [Member] | Level 2 [Member] | Pension Plan [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class [3] 35.6 36.3 35.6
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 [3] 35.6    
Cash and Cash Equivalents [Member] | Level 2 [Member] | Other Postretirement Benefit Plans [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class [4] 0.0 0.0 0.0
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 [4] 0.0    
Cash and Cash Equivalents [Member] | Level 3 [Member] | Pension Plan [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class [3] 0.0 0.0 0.0
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 [3] 0.0    
Cash and Cash Equivalents [Member] | Level 3 [Member] | Other Postretirement Benefit Plans [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class [4] 0.0 0.0 0.0
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 [4] 0.0    
Other - Pending Transactions and Accrued Income [Member] | Pension Plan [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class [5] 22.7 $ 8.3 $ 22.7
Year End Allocation      
Total [5]   0.20% 0.50%
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 [5] 22.7    
Other - Pending Transactions and Accrued Income [Member] | Other Postretirement Benefit Plans [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class [6] (2.9) $ (1.2) $ (2.9)
Year End Allocation      
Total [6]   (0.10%) (0.20%)
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 [6] (2.9)    
Other - Pending Transactions and Accrued Income [Member] | Other [Member] | Pension Plan [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class [5] 22.7 $ 8.3 $ 22.7
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 [5] 22.7    
Other - Pending Transactions and Accrued Income [Member] | Other [Member] | Other Postretirement Benefit Plans [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class [6] (2.9) (1.2) (2.9)
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 [6] (2.9)    
Other - Pending Transactions and Accrued Income [Member] | Level 1 [Member] | Pension Plan [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class [5] 0.0 0.0 0.0
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 [5] 0.0    
Other - Pending Transactions and Accrued Income [Member] | Level 1 [Member] | Other Postretirement Benefit Plans [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class [6] 0.0 0.0 0.0
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 [6] 0.0    
Other - Pending Transactions and Accrued Income [Member] | Level 2 [Member] | Pension Plan [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class [5] 0.0 0.0 0.0
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 [5] 0.0    
Other - Pending Transactions and Accrued Income [Member] | Level 2 [Member] | Other Postretirement Benefit Plans [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class [6] 0.0 0.0 0.0
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 [6] 0.0    
Other - Pending Transactions and Accrued Income [Member] | Level 3 [Member] | Pension Plan [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class [5] 0.0 0.0 0.0
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 [5] 0.0    
Other - Pending Transactions and Accrued Income [Member] | Level 3 [Member] | Other Postretirement Benefit Plans [Member]      
Pension and Other Postretirement Plans' Assets      
Asset Class [6] 0.0 $ 0.0 $ 0.0
Actual Return on Plan Assets      
Fair Value of Plan Assets as of December 31 [6] $ 0.0    
[1] The classification of Level 3 assets from the prior year was corrected in the current year presentation and included within the fair value hierarchy table as of December 31, 2017 as “Other” investments for which fair value is measured using net asset value per share in accordance with ASU 2015-07, Disclosure for Investments in Certain Entities That Calculate Net Asset Value per Share (or Its Equivalent). Management concluded that these disclosure errors were immaterial individually and in the aggregate to all prior periods presented.
[2] Includes investment securities loaned to borrowers under the securities lending program. See the “Investments Held in Trust for Future Liabilities” section of Note 1 for additional information.
[3] Amounts in “Other” column represent investments for which fair value is measured using net asset value per share.
[4] Amounts in “Other” column represent investments for which fair value is measured using net asset value per share.
[5] Amounts in “Other” column primarily represent accrued interest, dividend receivables and transactions pending settlement.
[6] Amounts in “Other” column primarily represent accrued interest, dividend receivables and transactions pending settlement.