Pension and Other Postretirement Benefit Plans (Fair Value of Plan Assets by Category) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Significant unobservable inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 106	$ 132
Fair Value, Measurements, Recurring | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3,317	3,174
Net asset value of plan assets	1,159	1,118
Fair Value, Measurements, Recurring | Pension Benefits | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	36	47
Fair Value, Measurements, Recurring | Pension Benefits | U.S. securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	439	455
Fair Value, Measurements, Recurring | Pension Benefits | International securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	117	111
Fair Value, Measurements, Recurring | Pension Benefits | Domestic Corporate Debt Securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	976	892
Fair Value, Measurements, Recurring | Pension Benefits | Foreign Corporate Debt Securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	153	138
Fair Value, Measurements, Recurring | Pension Benefits | U.S. Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	19	14
Fair Value, Measurements, Recurring | Pension Benefits | Foreign Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	235	186
Fair Value, Measurements, Recurring | Pension Benefits | Mutual funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	59	64
Fair Value, Measurements, Recurring | Pension Benefits | Common and Collective Funds, U.S. equity securities
Defined Benefit Plan Disclosure [Line Items]
Net asset value of plan assets	626	650
Fair Value, Measurements, Recurring | Pension Benefits | Common and Collective Funds, International equity securities
Defined Benefit Plan Disclosure [Line Items]
Net asset value of plan assets	70	56
Fair Value, Measurements, Recurring | Pension Benefits | Common and Collective Funds, Short-term investment fund
Defined Benefit Plan Disclosure [Line Items]
Net asset value of plan assets	33	30
Fair Value, Measurements, Recurring | Pension Benefits | Common and Collective Funds, International Debt Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Net asset value of plan assets	134	104
Fair Value, Measurements, Recurring | Pension Benefits | Common and Collective Fund of Funds, International Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Net asset value of plan assets	286	268
Fair Value, Measurements, Recurring | Pension Benefits | Common and Collective Funds, Real Estate [Member]
Defined Benefit Plan Disclosure [Line Items]
Net asset value of plan assets	10	10
Fair Value, Measurements, Recurring | Pension Benefits | U.S. private equity investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	86	104
Net asset value of plan assets	0	0
Fair Value, Measurements, Recurring | Pension Benefits | Diversified fund of funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	8	14
Fair Value, Measurements, Recurring | Pension Benefits | Emerging growth
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	12	14
Fair Value, Measurements, Recurring | Pension Benefits | All other investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	18	17
Fair Value, Measurements, Recurring | Pension Benefits | Quoted prices (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	349	311
Fair Value, Measurements, Recurring | Pension Benefits | Quoted prices (Level 1) | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	36	47
Fair Value, Measurements, Recurring | Pension Benefits | Quoted prices (Level 1) | U.S. securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Fair Value, Measurements, Recurring | Pension Benefits | Quoted prices (Level 1) | International securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Fair Value, Measurements, Recurring | Pension Benefits | Quoted prices (Level 1) | Domestic Corporate Debt Securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Fair Value, Measurements, Recurring | Pension Benefits | Quoted prices (Level 1) | Foreign Corporate Debt Securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Fair Value, Measurements, Recurring | Pension Benefits | Quoted prices (Level 1) | U.S. Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	19	14
Fair Value, Measurements, Recurring | Pension Benefits | Quoted prices (Level 1) | Foreign Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	235	186
Fair Value, Measurements, Recurring | Pension Benefits | Quoted prices (Level 1) | Mutual funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	59	64
Fair Value, Measurements, Recurring | Pension Benefits | Quoted prices (Level 1) | U.S. private equity investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Fair Value, Measurements, Recurring | Pension Benefits | Quoted prices (Level 1) | Diversified fund of funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Fair Value, Measurements, Recurring | Pension Benefits | Quoted prices (Level 1) | Emerging growth
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Fair Value, Measurements, Recurring | Pension Benefits | Quoted prices (Level 1) | All other investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Fair Value, Measurements, Recurring | Pension Benefits | Other significant observable inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,703	1,613
Fair Value, Measurements, Recurring | Pension Benefits | Other significant observable inputs (Level 2) | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Fair Value, Measurements, Recurring | Pension Benefits | Other significant observable inputs (Level 2) | U.S. securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	439	455
Fair Value, Measurements, Recurring | Pension Benefits | Other significant observable inputs (Level 2) | International securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	117	111
Fair Value, Measurements, Recurring | Pension Benefits | Other significant observable inputs (Level 2) | Domestic Corporate Debt Securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	976	892
Fair Value, Measurements, Recurring | Pension Benefits | Other significant observable inputs (Level 2) | Foreign Corporate Debt Securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	153	138
Fair Value, Measurements, Recurring | Pension Benefits | Other significant observable inputs (Level 2) | U.S. Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Fair Value, Measurements, Recurring | Pension Benefits | Other significant observable inputs (Level 2) | Foreign Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Fair Value, Measurements, Recurring | Pension Benefits | Other significant observable inputs (Level 2) | Mutual funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Fair Value, Measurements, Recurring | Pension Benefits | Other significant observable inputs (Level 2) | Diversified fund of funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Fair Value, Measurements, Recurring | Pension Benefits | Other significant observable inputs (Level 2) | Emerging growth
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Fair Value, Measurements, Recurring | Pension Benefits | Other significant observable inputs (Level 2) | All other investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	18	17
Fair Value, Measurements, Recurring | Pension Benefits | Significant unobservable inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	106	132
Fair Value, Measurements, Recurring | Pension Benefits | Significant unobservable inputs (Level 3) | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Fair Value, Measurements, Recurring | Pension Benefits | Significant unobservable inputs (Level 3) | U.S. securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Fair Value, Measurements, Recurring | Pension Benefits | Significant unobservable inputs (Level 3) | International securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Fair Value, Measurements, Recurring | Pension Benefits | Significant unobservable inputs (Level 3) | Domestic Corporate Debt Securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Fair Value, Measurements, Recurring | Pension Benefits | Significant unobservable inputs (Level 3) | Foreign Corporate Debt Securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Fair Value, Measurements, Recurring | Pension Benefits | Significant unobservable inputs (Level 3) | U.S. Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Fair Value, Measurements, Recurring | Pension Benefits | Significant unobservable inputs (Level 3) | Foreign Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Fair Value, Measurements, Recurring | Pension Benefits | Significant unobservable inputs (Level 3) | Mutual funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Fair Value, Measurements, Recurring | Pension Benefits | Significant unobservable inputs (Level 3) | U.S. private equity investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	86	104
Fair Value, Measurements, Recurring | Pension Benefits | Significant unobservable inputs (Level 3) | Diversified fund of funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	8	14
Fair Value, Measurements, Recurring | Pension Benefits | Significant unobservable inputs (Level 3) | Emerging growth
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	12	14
Fair Value, Measurements, Recurring | Pension Benefits | Significant unobservable inputs (Level 3) | All other investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 0	$ 0