Other Postretirement Benefits - Schedule of Fair Values of Plan Assets (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	$ 330	$ 273
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Funds for retiree health benefits	262	270
Investments (including funds for retiree health benefits)	984	924
Funds for retiree health benefits measured at NAV per share (l)(n)	36	37
Pending activities	19	14
Total fair value of plan net assets	1,039	975	$ 994	$ 1,084
Other Postretirement Benefits | Equity
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	420	391
Other Postretirement Benefits | Other Fixed Income Debt
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	286	250
Other Postretirement Benefits | Cash and Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	16	13
Other Postretirement Benefits | Total investments
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	722	654
Other Postretirement Benefits | Level 1
Defined Benefit Plan Disclosure [Line Items]
Funds for retiree health benefits	168	165
Investments (including funds for retiree health benefits)	168	165
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	94	105
Investments (including funds for retiree health benefits)	816	759
Other Postretirement Benefits | Level 2 | Equity
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	420	391
Other Postretirement Benefits | Level 2 | Other Fixed Income Debt
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	286	250
Other Postretirement Benefits | Level 2 | Cash and Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	16	13
Other Postretirement Benefits | Level 2 | Total investments
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	$ 722	$ 654