Pension and Other Postretirement Plans - Pension and Other Postretirement Plan Assets (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Level 3
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	$ 350	$ 383	$ 287
Pension Benefits
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	3,827	3,932		$ 3,355
Pension Benefits | Level 1
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	478	301
Pension Benefits | Level 2
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	751	1,091
Pension Benefits | Level 3
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	297	329
Pension Benefits | Assets Measured at NAV
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	2,301	2,211
Pension Benefits | Equity securities funds
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	1,394	1,406
Pension Benefits | Equity securities funds | Level 1
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	254	269
Pension Benefits | Equity securities funds | Level 2
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	106	133
Pension Benefits | Equity securities funds | Level 3
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Benefits | Equity securities funds | Assets Measured at NAV
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	1,034	1,004
Pension Benefits | Fixed-income securities
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	1,431	1,470
Pension Benefits | Fixed-income securities | Level 1
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Benefits | Fixed-income securities | Level 2
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	605	834
Pension Benefits | Fixed-income securities | Level 3
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	21	18
Pension Benefits | Fixed-income securities | Assets Measured at NAV
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	805	618
Pension Benefits | Alternatives
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	596	637
Pension Benefits | Alternatives | Level 1
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Benefits | Alternatives | Level 2
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Benefits | Alternatives | Level 3
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	134	139
Pension Benefits | Alternatives | Assets Measured at NAV
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	462	498
Pension Benefits | Other
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	406	419
Pension Benefits | Other | Level 1
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	224	32
Pension Benefits | Other | Level 2
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	40	124
Pension Benefits | Other | Level 3
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	142	172
Pension Benefits | Other | Assets Measured at NAV
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	0	91
Other Postretirement Benefits
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	53	54		$ 55
Other Postretirement Benefits | Deposit administration fund
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	53	54
Other Postretirement Benefits | Deposit administration fund | Level 1
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other Postretirement Benefits | Deposit administration fund | Level 2
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other Postretirement Benefits | Deposit administration fund | Level 3
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	53	54
Other Postretirement Benefits | Deposit administration fund | Assets Measured at NAV
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	$ 0	$ 0