Postretirement Benefits Other Than Pensions - Assumptions used to Determine Benefit Obligations (Details)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Benefit Obligation
Defined Benefit Plan Disclosure [Line Items]
Healthcare cost trend rate assumed for next year (as a percent)	6.52%	6.43%
Assumed decline in cost trend rate (as a percent)	4.50%	4.50%
Weighted average discount rate (as a percent)	3.20%	4.26%
Net Periodic Benefit Cost
Defined Benefit Plan Disclosure [Line Items]
Healthcare cost trend rate assumed for next year (as a percent)	6.43%	6.72%	7.00%
Assumed decline in cost trend rate (as a percent)	4.50%	4.50%	4.50%
Weighted average discount rate (as a percent)	4.26%	3.53%	3.97%
Effective rate for interest on benefit obligations (as a percent)	3.93%	3.16%	3.32%
Effective discount rate for service cost (as a percent)	4.46%	3.77%	4.44%
Effective rate for interest on service cost (as a percent)	4.29%	3.59%	4.08%