Other Postretirement Benefits - Schedule of Fair Values of Plan Assets (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	$ 465	$ 397
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	1,115	1,026	$ 885	$ 1,039
Other Postretirement Benefits | Fair Value, Inputs, Level 1 and 2 | Total investments
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	842	758
Other Postretirement Benefits | Fair Value, Inputs, Level 1 and 2 | Equity
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	448	404
Other Postretirement Benefits | Fair Value, Inputs, Level 1 and 2 | Other Fixed Income Debt
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	367	331
Other Postretirement Benefits | Fair Value, Inputs, Level 1 and 2 | Cash and Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	27	23
Other Postretirement Benefits | Fair Value, Inputs, Level 1 and 2 | Funds For Retiree Health Benefits
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	213	208
Other Postretirement Benefits | Fair Value, Inputs, Level 1 and 2 | Investments, Including Funds For Retiree Health Benefit
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	1,055	966
Other Postretirement Benefits | Fair Value, Inputs, Level 1 and 2 | Funds For Retiree Health Benefits Measured At Net Asset Value
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	41	42
Other Postretirement Benefits | Fair Value, Inputs, Level 1 and 2 | Pending Activities
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	19	18
Other Postretirement Benefits | Level 1 | Total investments
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	0	0
Other Postretirement Benefits | Level 1 | Equity
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	0	0
Other Postretirement Benefits | Level 1 | Other Fixed Income Debt
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	0	0
Other Postretirement Benefits | Level 1 | Cash and Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	0	0
Other Postretirement Benefits | Level 1 | Funds For Retiree Health Benefits
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	116	110
Other Postretirement Benefits | Level 1 | Investments, Including Funds For Retiree Health Benefit
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	116	110
Other Postretirement Benefits | Level 2 | Total investments
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	842	758
Other Postretirement Benefits | Level 2 | Equity
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	448	404
Other Postretirement Benefits | Level 2 | Other Fixed Income Debt
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	367	331
Other Postretirement Benefits | Level 2 | Cash and Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	27	23
Other Postretirement Benefits | Level 2 | Funds For Retiree Health Benefits
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	97	98
Other Postretirement Benefits | Level 2 | Investments, Including Funds For Retiree Health Benefit
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	$ 939	$ 856