Pension and Other Postretirement Benefit Plans (Fair Value of Plan Assets by Category) (Details) - USD ($)	Dec. 31, 2019	Dec. 31, 2018
Significant unobservable inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 66,000,000	$ 86,000,000
Fair Value, Measurements, Recurring | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3,527,000,000	3,194,000,000
Net asset value of plan assets	973,000,000	969,000,000
Fair Value, Measurements, Recurring | Pension Benefits | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	24,000,000	10,000,000
Fair Value, Measurements, Recurring | Pension Benefits | Government and government agency securities, U.S. securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	488,000,000	761,000,000
Fair Value, Measurements, Recurring | Pension Benefits | Government and government agency securities, international securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	97,000,000	97,000,000
Fair Value, Measurements, Recurring | Pension Benefits | Corporate bonds and notes, U.S. companies
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,389,000,000	860,000,000
Fair Value, Measurements, Recurring | Pension Benefits | Corporate bonds and notes, international companies
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	277,000,000	155,000,000
Fair Value, Measurements, Recurring | Pension Benefits | Equity securities, U.S. companies
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	18,000,000
Fair Value, Measurements, Recurring | Pension Benefits | Equity Securities, International Companies
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	51,000,000	185,000,000
Fair Value, Measurements, Recurring | Pension Benefits | Mutual funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	128,000,000	35,000,000
Fair Value, Measurements, Recurring | Pension Benefits | Common and Collective Funds, U.S. equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	367,000,000	501,000,000
Net asset value of plan assets	367,000,000	501,000,000
Fair Value, Measurements, Recurring | Pension Benefits | Common and Collective Funds, International equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	215,000,000	52,000,000
Net asset value of plan assets	215,000,000	52,000,000
Fair Value, Measurements, Recurring | Pension Benefits | Common and Collective Funds, Short-term investment fund
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	15,000,000	102,000,000
Net asset value of plan assets	15,000,000	102,000,000
Fair Value, Measurements, Recurring | Pension Benefits | Common and Collective Funds, International debt securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	251,000,000	209,000,000
Net asset value of plan assets	251,000,000	209,000,000
Fair Value, Measurements, Recurring | Pension Benefits | Common and Collective Fund of Funds, International equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	59,000,000	50,000,000
Net asset value of plan assets	59,000,000	50,000,000
Fair Value, Measurements, Recurring | Pension Benefits | Common and Collective Funds, Real estate
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	34,000,000	36,000,000
Net asset value of plan assets	34,000,000	36,000,000
Fair Value, Measurements, Recurring | Pension Benefits | Limited partnerships, U.S. private equity investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	53,000,000	68,000,000
Fair Value, Measurements, Recurring | Pension Benefits | Limited partnerships, diversified fund of funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	5,000,000	6,000,000
Fair Value, Measurements, Recurring | Pension Benefits | Limited partnerships, emerging growth
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	8,000,000	12,000,000
Fair Value, Measurements, Recurring | Pension Benefits | All other investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	66,000,000	37,000,000
Net asset value of plan assets	32,000,000	19,000,000
Fair Value, Measurements, Recurring | Pension Benefits | Quoted prices (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	51,000,000	238,000,000
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	0	0
Fair Value, Measurements, Recurring | Pension Benefits | Quoted prices (Level 1) | Government and government agency securities, 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) | Government and government agency securities, 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) | Corporate bonds and notes, U.S. companies
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Fair Value, Measurements, Recurring | Pension Benefits | Quoted prices (Level 1) | Corporate bonds and notes, international companies
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Fair Value, Measurements, Recurring | Pension Benefits | Quoted prices (Level 1) | Equity securities, U.S. companies
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	18,000,000
Fair Value, Measurements, Recurring | Pension Benefits | Quoted prices (Level 1) | Equity Securities, International Companies
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	51,000,000	185,000,000
Fair Value, Measurements, Recurring | Pension Benefits | Quoted prices (Level 1) | Mutual funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	35,000,000
Fair Value, Measurements, Recurring | Pension Benefits | Quoted prices (Level 1) | Limited partnerships, 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) | Limited partnerships, 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) | Limited partnerships, 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	2,437,000,000	1,901,000,000
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	24,000,000	10,000,000
Fair Value, Measurements, Recurring | Pension Benefits | Other significant observable inputs (Level 2) | Government and government agency securities, U.S. securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	488,000,000	761,000,000
Fair Value, Measurements, Recurring | Pension Benefits | Other significant observable inputs (Level 2) | Government and government agency securities, international securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	97,000,000	97,000,000
Fair Value, Measurements, Recurring | Pension Benefits | Other significant observable inputs (Level 2) | Corporate bonds and notes, U.S. companies
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,389,000,000	860,000,000
Fair Value, Measurements, Recurring | Pension Benefits | Other significant observable inputs (Level 2) | Corporate bonds and notes, international companies
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	277,000,000	155,000,000
Fair Value, Measurements, Recurring | Pension Benefits | Other significant observable inputs (Level 2) | Equity securities, U.S. companies
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) | Equity Securities, International Companies
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	128,000,000	0
Fair Value, Measurements, Recurring | Pension Benefits | Other significant observable inputs (Level 2) | Limited partnerships, U.S. private equity 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) | Limited partnerships, 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) | Limited partnerships, 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	34,000,000	18,000,000
Fair Value, Measurements, Recurring | Pension Benefits | Significant unobservable inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	66,000,000	86,000,000
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) | Government and government agency securities, 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) | Government and government agency securities, 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) | Corporate bonds and notes, U.S. companies
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Fair Value, Measurements, Recurring | Pension Benefits | Significant unobservable inputs (Level 3) | Corporate bonds and notes, international companies
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Fair Value, Measurements, Recurring | Pension Benefits | Significant unobservable inputs (Level 3) | Equity securities, U.S. companies
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Fair Value, Measurements, Recurring | Pension Benefits | Significant unobservable inputs (Level 3) | Equity Securities, International Companies
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) | Limited partnerships, U.S. private equity investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	53,000,000	68,000,000
Fair Value, Measurements, Recurring | Pension Benefits | Significant unobservable inputs (Level 3) | Limited partnerships, diversified fund of funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	5,000,000	6,000,000
Fair Value, Measurements, Recurring | Pension Benefits | Significant unobservable inputs (Level 3) | Limited partnerships, emerging growth
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	8,000,000	12,000,000
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