Postretirement Benefits Other than Pensions - Discount Rate to Determine the Benefit Obligation (Details)	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Benefit Obligation [Abstract]
Discount rate (as a percent)	1.70%	2.70%	3.83%