Benefit Plans - Fair Value of Pension Plan Assets (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Total investments at fair value	$ 1,902	$ 1,057	$ 816
Significant Unobservable Inputs (Level 3) | Corporate bonds and other fixed-income securities
Defined Benefit Plan Disclosure [Line Items]
Total investments at fair value	1,836	1,032	$ 790
Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Total investments at fair value	11,430	9,908
Pension Plans | Total Fair Value
Defined Benefit Plan Disclosure [Line Items]
Total investments at fair value	9,133	7,915
Pension Plans | Total Fair Value | U.S. equity securities
Defined Benefit Plan Disclosure [Line Items]
Total investments at fair value	2	2
Pension Plans | Total Fair Value | Non-U.S. equity securities
Defined Benefit Plan Disclosure [Line Items]
Total investments at fair value	2	5
Pension Plans | Total Fair Value | Pooled funds - equity securities
Defined Benefit Plan Disclosure [Line Items]
Total investments at fair value	2,186	1,951
Pension Plans | Total Fair Value | Total equity securities
Defined Benefit Plan Disclosure [Line Items]
Total investments at fair value	2,190	1,958
Pension Plans | Total Fair Value | Government bonds
Defined Benefit Plan Disclosure [Line Items]
Total investments at fair value	3,328	3,156
Pension Plans | Total Fair Value | Pooled funds - fixed-income securities
Defined Benefit Plan Disclosure [Line Items]
Total investments at fair value	575	573
Pension Plans | Total Fair Value | Corporate bonds and other fixed-income securities
Defined Benefit Plan Disclosure [Line Items]
Total investments at fair value	2,727	2,050
Pension Plans | Total Fair Value | Total fixed-income securities
Defined Benefit Plan Disclosure [Line Items]
Total investments at fair value	6,630	5,779
Pension Plans | Total Fair Value | Real estate
Defined Benefit Plan Disclosure [Line Items]
Total investments at fair value	186	130
Pension Plans | Total Fair Value | Private equity
Defined Benefit Plan Disclosure [Line Items]
Total investments at fair value	3	2
Pension Plans | Total Fair Value | Cash
Defined Benefit Plan Disclosure [Line Items]
Total investments at fair value	122	44
Pension Plans | Total Fair Value | Other
Defined Benefit Plan Disclosure [Line Items]
Total investments at fair value	2	2
Pension Plans | Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Total investments at fair value	1,672	1,469
Pension Plans | Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. equity securities
Defined Benefit Plan Disclosure [Line Items]
Total investments at fair value	2	2
Pension Plans | Quoted Prices in Active Markets for Identical Assets (Level 1) | Non-U.S. equity securities
Defined Benefit Plan Disclosure [Line Items]
Total investments at fair value	2	5
Pension Plans | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pooled funds - equity securities
Defined Benefit Plan Disclosure [Line Items]
Total investments at fair value	890	743
Pension Plans | Quoted Prices in Active Markets for Identical Assets (Level 1) | Total equity securities
Defined Benefit Plan Disclosure [Line Items]
Total investments at fair value	894	750
Pension Plans | Quoted Prices in Active Markets for Identical Assets (Level 1) | Government bonds
Defined Benefit Plan Disclosure [Line Items]
Total investments at fair value	53	62
Pension Plans | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pooled funds - fixed-income securities
Defined Benefit Plan Disclosure [Line Items]
Total investments at fair value	417	429
Pension Plans | Quoted Prices in Active Markets for Identical Assets (Level 1) | Corporate bonds and other fixed-income securities
Defined Benefit Plan Disclosure [Line Items]
Total investments at fair value	66	87
Pension Plans | Quoted Prices in Active Markets for Identical Assets (Level 1) | Total fixed-income securities
Defined Benefit Plan Disclosure [Line Items]
Total investments at fair value	536	578
Pension Plans | Quoted Prices in Active Markets for Identical Assets (Level 1) | Real estate
Defined Benefit Plan Disclosure [Line Items]
Total investments at fair value	124	108
Pension Plans | Quoted Prices in Active Markets for Identical Assets (Level 1) | Private equity
Defined Benefit Plan Disclosure [Line Items]
Total investments at fair value	0	0
Pension Plans | Quoted Prices in Active Markets for Identical Assets (Level 1) | Cash
Defined Benefit Plan Disclosure [Line Items]
Total investments at fair value	117	32
Pension Plans | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other
Defined Benefit Plan Disclosure [Line Items]
Total investments at fair value	1	1
Pension Plans | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Total investments at fair value	5,559	5,389
Pension Plans | Significant Other Observable Inputs (Level 2) | U.S. equity securities
Defined Benefit Plan Disclosure [Line Items]
Total investments at fair value	0	0
Pension Plans | Significant Other Observable Inputs (Level 2) | Non-U.S. equity securities
Defined Benefit Plan Disclosure [Line Items]
Total investments at fair value	0	0
Pension Plans | Significant Other Observable Inputs (Level 2) | Pooled funds - equity securities
Defined Benefit Plan Disclosure [Line Items]
Total investments at fair value	1,296	1,208
Pension Plans | Significant Other Observable Inputs (Level 2) | Total equity securities
Defined Benefit Plan Disclosure [Line Items]
Total investments at fair value	1,296	1,208
Pension Plans | Significant Other Observable Inputs (Level 2) | Government bonds
Defined Benefit Plan Disclosure [Line Items]
Total investments at fair value	3,275	3,094
Pension Plans | Significant Other Observable Inputs (Level 2) | Pooled funds - fixed-income securities
Defined Benefit Plan Disclosure [Line Items]
Total investments at fair value	158	144
Pension Plans | Significant Other Observable Inputs (Level 2) | Corporate bonds and other fixed-income securities
Defined Benefit Plan Disclosure [Line Items]
Total investments at fair value	825	931
Pension Plans | Significant Other Observable Inputs (Level 2) | Total fixed-income securities
Defined Benefit Plan Disclosure [Line Items]
Total investments at fair value	4,258	4,169
Pension Plans | Significant Other Observable Inputs (Level 2) | Real estate
Defined Benefit Plan Disclosure [Line Items]
Total investments at fair value	0	0
Pension Plans | Significant Other Observable Inputs (Level 2) | Private equity
Defined Benefit Plan Disclosure [Line Items]
Total investments at fair value	0	0
Pension Plans | Significant Other Observable Inputs (Level 2) | Cash
Defined Benefit Plan Disclosure [Line Items]
Total investments at fair value	5	12
Pension Plans | Significant Other Observable Inputs (Level 2) | Other
Defined Benefit Plan Disclosure [Line Items]
Total investments at fair value	0	0
Pension Plans | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Total investments at fair value	1,902	1,057
Pension Plans | Significant Unobservable Inputs (Level 3) | U.S. equity securities
Defined Benefit Plan Disclosure [Line Items]
Total investments at fair value	0	0
Pension Plans | Significant Unobservable Inputs (Level 3) | Non-U.S. equity securities
Defined Benefit Plan Disclosure [Line Items]
Total investments at fair value	0	0
Pension Plans | Significant Unobservable Inputs (Level 3) | Pooled funds - equity securities
Defined Benefit Plan Disclosure [Line Items]
Total investments at fair value	0	0
Pension Plans | Significant Unobservable Inputs (Level 3) | Total equity securities
Defined Benefit Plan Disclosure [Line Items]
Total investments at fair value	0	0
Pension Plans | Significant Unobservable Inputs (Level 3) | Government bonds
Defined Benefit Plan Disclosure [Line Items]
Total investments at fair value	0	0
Pension Plans | Significant Unobservable Inputs (Level 3) | Pooled funds - fixed-income securities
Defined Benefit Plan Disclosure [Line Items]
Total investments at fair value	0	0
Pension Plans | Significant Unobservable Inputs (Level 3) | Corporate bonds and other fixed-income securities
Defined Benefit Plan Disclosure [Line Items]
Total investments at fair value	1,836	1,032
Pension Plans | Significant Unobservable Inputs (Level 3) | Total fixed-income securities
Defined Benefit Plan Disclosure [Line Items]
Total investments at fair value	1,836	1,032
Pension Plans | Significant Unobservable Inputs (Level 3) | Real estate
Defined Benefit Plan Disclosure [Line Items]
Total investments at fair value	62	22
Pension Plans | Significant Unobservable Inputs (Level 3) | Private equity
Defined Benefit Plan Disclosure [Line Items]
Total investments at fair value	3	2
Pension Plans | Significant Unobservable Inputs (Level 3) | Cash
Defined Benefit Plan Disclosure [Line Items]
Total investments at fair value	0	0
Pension Plans | Significant Unobservable Inputs (Level 3) | Other
Defined Benefit Plan Disclosure [Line Items]
Total investments at fair value	1	1
Pension Plans | Total assets in the fair value hierarchy
Defined Benefit Plan Disclosure [Line Items]
Total investments at fair value	$ 2,297	$ 1,993