Retirement Benefits - Pension and Other Postretirement Benefit Plan Target Asset Allocations (Details)	Dec. 31, 2020	Dec. 31, 2019
Pension Plan, Defined Benefit [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Target asset allocation percentage	100.00%	100.00%
Pension Plan, Defined Benefit [Member] | Equity Securities [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Target asset allocation percentage	34.00%	33.00%
Pension Plan, Defined Benefit [Member] | Fixed Income Securities [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Target asset allocation percentage	43.00%	44.00%
Pension Plan, Defined Benefit [Member] | Alternative Investments [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Target asset allocation percentage	23.00%	23.00%
Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Target asset allocation percentage	100.00%	100.00%
Other Postretirement Benefit Plan, Defined Benefit [Member] | Equity Securities [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Target asset allocation percentage	45.00%	46.00%
Other Postretirement Benefit Plan, Defined Benefit [Member] | Fixed Income Securities [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Target asset allocation percentage	39.00%	32.00%
Other Postretirement Benefit Plan, Defined Benefit [Member] | Alternative Investments [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Target asset allocation percentage	16.00%	22.00%