Employee Retirement Benefits - Fair Value of Pension Plan Assets (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Fair value measurements of plan assets [Abstract]
Fair value	$ 4,800	$ 3,806
Reported Value Measurement [Member]
Fair value measurements of plan assets [Abstract]
Fair value	3,670	2,734
Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1) [Member] | Reported Value Measurement [Member]
Fair value measurements of plan assets [Abstract]
Fair value	1,767	1,131
Significant Other Observable Inputs (Level 2) [Member] | Reported Value Measurement [Member]
Fair value measurements of plan assets [Abstract]
Fair value	1,900	1,601
Significant Unobservable Inputs (Level 3) [Member] | Reported Value Measurement [Member]
Fair value measurements of plan assets [Abstract]
Fair value	3	2
Equity Securities [Member]
Fair value measurements of plan assets [Abstract]
Foreign investments	741	459
Equity Securities [Member] | Reported Value Measurement [Member]
Fair value measurements of plan assets [Abstract]
Fair value	1,605	1,043
Equity Securities [Member] | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1) [Member] | Reported Value Measurement [Member]
Fair value measurements of plan assets [Abstract]
Fair value	1,593	1,030
Equity Securities [Member] | Significant Other Observable Inputs (Level 2) [Member] | Reported Value Measurement [Member]
Fair value measurements of plan assets [Abstract]
Fair value	9	11
Equity Securities [Member] | Significant Unobservable Inputs (Level 3) [Member] | Reported Value Measurement [Member]
Fair value measurements of plan assets [Abstract]
Fair value	3	2
Equity Commingled Vehicles [Member]
Fair value measurements of plan assets [Abstract]
Foreign investments	141	193
Equity Commingled Vehicles [Member] | Reported Value Measurement [Member]
Fair value measurements of plan assets [Abstract]
Fair value	706	638
Equity Commingled Vehicles [Member] | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1) [Member] | Reported Value Measurement [Member]
Fair value measurements of plan assets [Abstract]
Fair value	0	0
Equity Commingled Vehicles [Member] | Significant Other Observable Inputs (Level 2) [Member] | Reported Value Measurement [Member]
Fair value measurements of plan assets [Abstract]
Fair value	706	638
Equity Commingled Vehicles [Member] | Significant Unobservable Inputs (Level 3) [Member] | Reported Value Measurement [Member]
Fair value measurements of plan assets [Abstract]
Fair value	0	0
U.S. Government And Municipal Bonds [Member] | Reported Value Measurement [Member]
Fair value measurements of plan assets [Abstract]
Fair value	102	95
U.S. Government And Municipal Bonds [Member] | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1) [Member] | Reported Value Measurement [Member]
Fair value measurements of plan assets [Abstract]
Fair value	95	84
U.S. Government And Municipal Bonds [Member] | Significant Other Observable Inputs (Level 2) [Member] | Reported Value Measurement [Member]
Fair value measurements of plan assets [Abstract]
Fair value	7	11
U.S. Government And Municipal Bonds [Member] | Significant Unobservable Inputs (Level 3) [Member] | Reported Value Measurement [Member]
Fair value measurements of plan assets [Abstract]
Fair value	0	0
Corporate debt securities [Member]
Fair value measurements of plan assets [Abstract]
Foreign investments	76	77
Corporate debt securities [Member] | Reported Value Measurement [Member]
Fair value measurements of plan assets [Abstract]
Fair value	247	252
Corporate debt securities [Member] | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1) [Member] | Reported Value Measurement [Member]
Fair value measurements of plan assets [Abstract]
Fair value	0	0
Corporate debt securities [Member] | Significant Other Observable Inputs (Level 2) [Member] | Reported Value Measurement [Member]
Fair value measurements of plan assets [Abstract]
Fair value	247	252
Corporate debt securities [Member] | Significant Unobservable Inputs (Level 3) [Member] | Reported Value Measurement [Member]
Fair value measurements of plan assets [Abstract]
Fair value	0	0
Asset-backed securities [Member] | Reported Value Measurement [Member]
Fair value measurements of plan assets [Abstract]
Fair value	416	253
Asset-backed securities [Member] | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1) [Member] | Reported Value Measurement [Member]
Fair value measurements of plan assets [Abstract]
Fair value	0	0
Asset-backed securities [Member] | Significant Other Observable Inputs (Level 2) [Member] | Reported Value Measurement [Member]
Fair value measurements of plan assets [Abstract]
Fair value	416	253
Asset-backed securities [Member] | Significant Unobservable Inputs (Level 3) [Member] | Reported Value Measurement [Member]
Fair value measurements of plan assets [Abstract]
Fair value	0	0
Debt Security Commingled Vehicles [Member]
Fair value measurements of plan assets [Abstract]
Foreign investments	5
Debt Security Commingled Vehicles [Member] | Reported Value Measurement [Member]
Fair value measurements of plan assets [Abstract]
Fair value	190	133
Debt Security Commingled Vehicles [Member] | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1) [Member] | Reported Value Measurement [Member]
Fair value measurements of plan assets [Abstract]
Fair value	47	0
Debt Security Commingled Vehicles [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair value measurements of plan assets [Abstract]
Fair value	143	133
Debt Security Commingled Vehicles [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair value measurements of plan assets [Abstract]
Fair value	0	0
Convertible securities [Member]
Fair value measurements of plan assets [Abstract]
Foreign investments	20	30
Convertible securities [Member] | Reported Value Measurement [Member]
Fair value measurements of plan assets [Abstract]
Fair value	404	320
Convertible securities [Member] | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1) [Member] | Reported Value Measurement [Member]
Fair value measurements of plan assets [Abstract]
Fair value	32	17
Convertible securities [Member] | Significant Other Observable Inputs (Level 2) [Member] | Reported Value Measurement [Member]
Fair value measurements of plan assets [Abstract]
Fair value	372	303
Convertible securities [Member] | Significant Unobservable Inputs (Level 3) [Member] | Reported Value Measurement [Member]
Fair value measurements of plan assets [Abstract]
Fair value	0	0
Investments measured at net asset value [Member]
Fair value measurements of plan assets [Abstract]
Foreign investments	190
Investments measured at net asset value [Member] | Reported Value Measurement [Member]
Fair value measurements of plan assets [Abstract]
Fair value	$ 1,130	1,072
Foreign investments		$ 214