Other Postretirement Benefits - Schedule of Fair Values of Plan Assets (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	$ 326	$ 330
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Funds for retiree health benefits	204	262
Investments (including funds for retiree health benefits)	829	984
Funds for retiree health benefits measured at NAV per share	33	36
Pending activities	23	19
Total fair value of plan net assets	885	1,039	$ 975	$ 994
Other Postretirement Benefits | Equity
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	322	420
Other Postretirement Benefits | Other Fixed Income Debt
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	289	286
Other Postretirement Benefits | Cash and Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	14	16
Other Postretirement Benefits | Total investments
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	625	722
Other Postretirement Benefits | Level 1
Defined Benefit Plan Disclosure [Line Items]
Funds for retiree health benefits	118	168
Investments (including funds for retiree health benefits)	118	168
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 | Total investments
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	0	0
Other Postretirement Benefits | Level 2
Defined Benefit Plan Disclosure [Line Items]
Funds for retiree health benefits	86	94
Investments (including funds for retiree health benefits)	711	816
Other Postretirement Benefits | Level 2 | Equity
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	322	420
Other Postretirement Benefits | Level 2 | Other Fixed Income Debt
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	289	286
Other Postretirement Benefits | Level 2 | Cash and Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	14	16
Other Postretirement Benefits | Level 2 | Total investments
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	$ 625	$ 722