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 |