Other Postretirement Benefits - Schedule of Fair Values of Plan Assets (Details) - Other Postretirement Benefits - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Defined Benefit Plan Disclosure [Line Items]
Funds for retiree health benefits	$ 270	$ 282
Investments (including funds for retiree health benefits)	924	942
Funds for retiree health benefits measured at NAV per share (l)(n)	37	43
Pending activities	14	9
Total fair value of plan net assets	975	994	$ 1,084	$ 1,113
Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	391	393
Other Fixed Income Debt
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	250	260
Cash and Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	13	7
Total investments
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	654	660
Level 1
Defined Benefit Plan Disclosure [Line Items]
Funds for retiree health benefits	165	162
Investments (including funds for retiree health benefits)	165	162
Level 1 | Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	0	0
Level 1 | Other Fixed Income Debt
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	0	0
Level 1 | Cash and Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	0	0
Level 1 | Total investments
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	0	0
Level 2
Defined Benefit Plan Disclosure [Line Items]
Funds for retiree health benefits	105	120
Investments (including funds for retiree health benefits)	759	780
Level 2 | Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	391	393
Level 2 | Other Fixed Income Debt
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	250	260
Level 2 | Cash and Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	13	7
Level 2 | Total investments
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	$ 654	$ 660