Pension and Other Postretirement Benefit Plans (Fair Value of Plan Assets by Category) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Significant unobservable inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 86	$ 106
Fair Value, Measurements, Recurring | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3,194	3,317
Net asset value of plan assets	969	1,159
Fair Value, Measurements, Recurring | Pension Benefits | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	10	36
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	761	439
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	117
Fair Value, Measurements, Recurring | Pension Benefits | Corporate bonds and notes, U.S. companies
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	860	976
Fair Value, Measurements, Recurring | Pension Benefits | Corporate bonds and notes, international companies
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	155	153
Fair Value, Measurements, Recurring | Pension Benefits | Equity securities, U.S. companies
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	18	19
Fair Value, Measurements, Recurring | Pension Benefits | Equity securities, international companies
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	185	235
Fair Value, Measurements, Recurring | Pension Benefits | Mutual funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	35	59
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	501	689
Net asset value of plan assets	501	689
Fair Value, Measurements, Recurring | Pension Benefits | Common and Collective Funds, International equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	52	70
Net asset value of plan assets	52	70
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	102	70
Net asset value of plan assets	102	70
Fair Value, Measurements, Recurring | Pension Benefits | Common and Collective Funds, International debt securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	209	229
Net asset value of plan assets	209	229
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	50	46
Net asset value of plan assets	50	46
Fair Value, Measurements, Recurring | Pension Benefits | Common and Collective Funds, Real estate
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	36	44
Net asset value of plan assets	36	44
Fair Value, Measurements, Recurring | Pension Benefits | Limited partnerships, U.S. private equity investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	68	86
Fair Value, Measurements, Recurring | Pension Benefits | Limited partnerships, diversified fund of funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	6	8
Fair Value, Measurements, Recurring | Pension Benefits | Limited partnerships, emerging growth
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	12	12
Fair Value, Measurements, Recurring | Pension Benefits | All other investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	37	29
Net asset value of plan assets	19	11
Fair Value, Measurements, Recurring | Pension Benefits | Quoted prices (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	238	313
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	18	19
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	185	235
Fair Value, Measurements, Recurring | Pension Benefits | Quoted prices (Level 1) | Mutual funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	35	59
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	1,901	1,739
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	10	36
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	761	439
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	117
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	860	976
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	155	153
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	0	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	18	18
Fair Value, Measurements, Recurring | Pension Benefits | Significant unobservable inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	86	106
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	68	86
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	6	8
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	12	12
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