Pension and Other Postretirement Benefit Plans - Recurring Fair Value Measures, Activity in Level 3 (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Pension [Member]
Activity in Level 3 [Roll Forward]
Fair Value, Beginning of Year	$ 699.6	$ 598.0
Fair Value, End of Year	759.4	699.6
Pension [Member] | Level 3 [Member]
Activity in Level 3 [Roll Forward]
Fair Value, Beginning of Year	26.0
Fair Value, End of Year	25.0	26.0
Pension [Member] | Private Equity [Member]
Activity in Level 3 [Roll Forward]
Fair Value, Beginning of Year	8.0
Fair Value, End of Year	7.0	8.0
Pension [Member] | Private Equity [Member] | Level 3 [Member]
Activity in Level 3 [Roll Forward]
Fair Value, Beginning of Year	8.0	27.8
Defined Benefit Plan, Plan Assets Level 3 Reconciliation, Increase (Decrease) for Actual Return (Loss) on Plan Assets Still Held	0.1	0.4
Defined Benefit Plan, Plan Assets Level 3 Reconciliation, Increase (Decrease) for Purchase, Sale, and Settlement	(1.1)	(20.2)
Fair Value, End of Year	7.0	8.0
Pension [Member] | Real Estate [Member]
Activity in Level 3 [Roll Forward]
Fair Value, Beginning of Year	18.0
Fair Value, End of Year	18.0	18.0
Pension [Member] | Real Estate [Member] | Level 3 [Member]
Activity in Level 3 [Roll Forward]
Fair Value, Beginning of Year	18.0	20.8
Defined Benefit Plan, Plan Assets Level 3 Reconciliation, Increase (Decrease) for Actual Return (Loss) on Plan Assets Still Held	0.0	(1.3)
Defined Benefit Plan, Plan Assets Level 3 Reconciliation, Increase (Decrease) for Purchase, Sale, and Settlement	0.0	(1.5)
Fair Value, End of Year	18.0	18.0
Postretirement Health and Life [Member]
Activity in Level 3 [Roll Forward]
Fair Value, Beginning of Year	173.7	154.3
Fair Value, End of Year	186.0	173.7
Postretirement Health and Life [Member] | Level 3 [Member]
Activity in Level 3 [Roll Forward]
Fair Value, Beginning of Year	1.7
Fair Value, End of Year	1.7	1.7
Postretirement Health and Life [Member] | Private Equity [Member]
Activity in Level 3 [Roll Forward]
Fair Value, Beginning of Year	1.7
Fair Value, End of Year	1.7	1.7
Postretirement Health and Life [Member] | Private Equity [Member] | Level 3 [Member]
Activity in Level 3 [Roll Forward]
Fair Value, Beginning of Year	1.7	6.5
Defined Benefit Plan, Plan Assets Level 3 Reconciliation, Increase (Decrease) for Actual Return (Loss) on Plan Assets Still Held	0.0	0.7
Defined Benefit Plan, Plan Assets Level 3 Reconciliation, Increase (Decrease) for Purchase, Sale, and Settlement	0.0	(5.5)
Fair Value, End of Year	$ 1.7	$ 1.7