Pension and Other Postretirement Benefit Plans - Fair Values of Other Postretirement Benefit Plan Assets (Details) - Other Postretirement Benefits - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]
Net assets in fair value hierarchy	$ 540	$ 1,246
Investments measured at NAV practical expedient	479	667
Plan assets at fair value	1,019	1,913	$ 1,984
Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	125	65
Developed markets equities, Investment funds
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	48	53
Emerging markets equities, Investment funds
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	10	29
Government and agency obligations, Investment funds
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	1	2
Developed markets, Equity securities
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	231	229
Government and agency obligations, Fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	43	339
Corporate obligations, Fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	60	311
Mortgage and asset backed-securities, Fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	22	218
Quoted Prices In Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Net assets in fair value hierarchy	415	378
Quoted Prices In Active Markets for Identical Assets (Level 1) | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	125	65
Quoted Prices In Active Markets for Identical Assets (Level 1) | Developed markets equities, Investment funds
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	48	53
Quoted Prices In Active Markets for Identical Assets (Level 1) | Emerging markets equities, Investment funds
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	10	29
Quoted Prices In Active Markets for Identical Assets (Level 1) | Government and agency obligations, Investment funds
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	1	2
Quoted Prices In Active Markets for Identical Assets (Level 1) | Developed markets, Equity securities
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	231	229
Quoted Prices In Active Markets for Identical Assets (Level 1) | Government and agency obligations, Fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	0	0
Quoted Prices In Active Markets for Identical Assets (Level 1) | Corporate obligations, Fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	0	0
Quoted Prices In Active Markets for Identical Assets (Level 1) | Mortgage and asset backed-securities, Fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	0	0
Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Net assets in fair value hierarchy	125	868
Significant Other Observable Inputs (Level 2) | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	0	0
Significant Other Observable Inputs (Level 2) | Developed markets equities, Investment funds
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	0	0
Significant Other Observable Inputs (Level 2) | Emerging markets equities, Investment funds
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	0	0
Significant Other Observable Inputs (Level 2) | Government and agency obligations, Investment funds
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	0	0
Significant Other Observable Inputs (Level 2) | Developed markets, Equity securities
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	0	0
Significant Other Observable Inputs (Level 2) | Government and agency obligations, Fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	43	339
Significant Other Observable Inputs (Level 2) | Corporate obligations, Fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	60	311
Significant Other Observable Inputs (Level 2) | Mortgage and asset backed-securities, Fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	22	218
Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Net assets in fair value hierarchy	0	0
Significant Unobservable Inputs (Level 3) | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	0	0
Significant Unobservable Inputs (Level 3) | Developed markets equities, Investment funds
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	0	0
Significant Unobservable Inputs (Level 3) | Emerging markets equities, Investment funds
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	0	0
Significant Unobservable Inputs (Level 3) | Government and agency obligations, Investment funds
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	0	0
Significant Unobservable Inputs (Level 3) | Developed markets, Equity securities
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	0	0
Significant Unobservable Inputs (Level 3) | Government and agency obligations, Fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	0	0
Significant Unobservable Inputs (Level 3) | Corporate obligations, Fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	0	0
Significant Unobservable Inputs (Level 3) | Mortgage and asset backed-securities, Fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	$ 0	$ 0