Pension and Other Postretirement Plans - Pension and Other Postretirement Plan Assets (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Level 3
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	$ 409	$ 350	$ 383
Pension Benefits
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	4,964	3,827	3,932
Pension Benefits | Level 1
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	513	478
Pension Benefits | Level 2
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	825	751
Pension Benefits | Level 3
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	357	297
Pension Benefits | Assets Measured at NAV
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	3,269	2,301
Pension Benefits | Equity securities funds
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	1,957	1,457
Pension Benefits | Equity securities funds | Level 1
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	47	254
Pension Benefits | Equity securities funds | Level 2
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	117	106
Pension Benefits | Equity securities funds | Level 3
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	71	63
Pension Benefits | Equity securities funds | Assets Measured at NAV
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	1,722	1,034
Pension Benefits | Fixed-income securities
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	1,732	1,520
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	687	628
Pension Benefits | Fixed-income securities | Level 3
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	69	87
Pension Benefits | Fixed-income securities | Assets Measured at NAV
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	976	805
Pension Benefits | Alternatives
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	776	596
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	205	134
Pension Benefits | Alternatives | Assets Measured at NAV
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	571	462
Pension Benefits | Other
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	499	254
Pension Benefits | Other | Level 1
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	466	224
Pension Benefits | Other | Level 2
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	21	17
Pension Benefits | Other | Level 3
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	12	13
Pension Benefits | Other | Assets Measured at NAV
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other Postretirement Benefits
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	52	53	$ 54
Other Postretirement Benefits | Deposit administration fund
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	52	53
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	52	53
Other Postretirement Benefits | Deposit administration fund | Assets Measured at NAV
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	$ 0	$ 0