Employee Retirement Benefits - Fair Value of Pension Plan Assets (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Fair Value Measured at Net Asset Value Per Share
Fair value measurements of plan assets [Abstract]
Foreign investments	$ 153	$ 190
Equity Securities [Member]
Fair value measurements of plan assets [Abstract]
Foreign investments	881	741
Equity Securities [Member] | Reported Value Measurement [Member]
Fair value measurements of plan assets [Abstract]
Fair value	2,030	1,605
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	2,017	1,593
Equity Securities [Member] | Significant Other Observable Inputs (Level 2) [Member] | Reported Value Measurement [Member]
Fair value measurements of plan assets [Abstract]
Fair value	10	9
Equity Securities [Member] | Significant Unobservable Inputs (Level 3) [Member] | Reported Value Measurement [Member]
Fair value measurements of plan assets [Abstract]
Fair value	3	3
Equity Commingled Vehicles [Member]
Fair value measurements of plan assets [Abstract]
Foreign investments	156	141
Equity Commingled Vehicles [Member] | Reported Value Measurement [Member]
Fair value measurements of plan assets [Abstract]
Fair value	668	706
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	668	706
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	177	102
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	169	95
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	8	7
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	93	76
Corporate debt securities [Member] | Reported Value Measurement [Member]
Fair value measurements of plan assets [Abstract]
Fair value	340	247
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	340	247
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	375	416
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	375	416
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	201	190
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	0	47
Debt Security Commingled Vehicles [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair value measurements of plan assets [Abstract]
Fair value	201	143
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	35	5
Convertible securities [Member] | Reported Value Measurement [Member]
Fair value measurements of plan assets [Abstract]
Fair value	517	404
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	64	32
Convertible securities [Member] | Significant Other Observable Inputs (Level 2) [Member] | Reported Value Measurement [Member]
Fair value measurements of plan assets [Abstract]
Fair value	453	372
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		20
Total Investments
Fair value measurements of plan assets [Abstract]
Fair value	5,314	4,800
Total Investments | Reported Value Measurement [Member]
Fair value measurements of plan assets [Abstract]
Fair value	4,308	3,670
Total Investments | 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	2,250	1,767
Total Investments | Significant Other Observable Inputs (Level 2) [Member] | Reported Value Measurement [Member]
Fair value measurements of plan assets [Abstract]
Fair value	2,055	1,900
Total Investments | Significant Unobservable Inputs (Level 3) [Member] | Reported Value Measurement [Member]
Fair value measurements of plan assets [Abstract]
Fair value	3	3
Total Investments | Fair Value Measured at Net Asset Value Per Share | Reported Value Measurement [Member]
Fair value measurements of plan assets [Abstract]
Fair value	$ 1,006	$ 1,130