Employee Retirement Benefits - Fair Value of Pension Plan Assets (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Fair value measurements of plan assets [Abstract]
Fair value	$ 3,806	$ 4,020
Reported Value Measurement [Member]
Fair value measurements of plan assets [Abstract]
Fair value	2,734	2,981
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	1,131	1,217
Significant Other Observable Inputs (Level 2) [Member] | Reported Value Measurement [Member]
Fair value measurements of plan assets [Abstract]
Fair value	1,601	1,752
Significant Unobservable Inputs (Level 3) [Member] | Reported Value Measurement [Member]
Fair value measurements of plan assets [Abstract]
Fair value	2	12
Equity Securities [Member]
Fair value measurements of plan assets [Abstract]
Foreign investments	459	480
Equity Securities [Member] | Reported Value Measurement [Member]
Fair value measurements of plan assets [Abstract]
Fair value	1,043	1,095
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	1,030	1,077
Equity Securities [Member] | Significant Other Observable Inputs (Level 2) [Member] | Reported Value Measurement [Member]
Fair value measurements of plan assets [Abstract]
Fair value	11	16
Equity Securities [Member] | Significant Unobservable Inputs (Level 3) [Member] | Reported Value Measurement [Member]
Fair value measurements of plan assets [Abstract]
Fair value	2	2
Equity Commingled Vehicles [Member]
Fair value measurements of plan assets [Abstract]
Foreign investments	193	287
Equity Commingled Vehicles [Member] | Reported Value Measurement [Member]
Fair value measurements of plan assets [Abstract]
Fair value	638	853
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	638	853
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	95	131
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	84	118
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	11	13
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	77	73
Corporate debt securities [Member] | Reported Value Measurement [Member]
Fair value measurements of plan assets [Abstract]
Fair value	252	251
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	3
Corporate debt securities [Member] | Significant Other Observable Inputs (Level 2) [Member] | Reported Value Measurement [Member]
Fair value measurements of plan assets [Abstract]
Fair value	252	238
Corporate debt securities [Member] | Significant Unobservable Inputs (Level 3) [Member] | Reported Value Measurement [Member]
Fair value measurements of plan assets [Abstract]
Fair value	0	10
Asset-backed securities [Member] | Reported Value Measurement [Member]
Fair value measurements of plan assets [Abstract]
Fair value	253	170
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	253	170
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		2
Debt Security Commingled Vehicles [Member] | Reported Value Measurement [Member]
Fair value measurements of plan assets [Abstract]
Fair value	133	155
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	0
Debt Security Commingled Vehicles [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair value measurements of plan assets [Abstract]
Fair value	133	155
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	30	35
Convertible securities [Member] | Reported Value Measurement [Member]
Fair value measurements of plan assets [Abstract]
Fair value	320	326
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	17	19
Convertible securities [Member] | Significant Other Observable Inputs (Level 2) [Member] | Reported Value Measurement [Member]
Fair value measurements of plan assets [Abstract]
Fair value	303	307
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	214	233
Investments measured at net asset value [Member] | Reported Value Measurement [Member]
Fair value measurements of plan assets [Abstract]
Fair value	$ 1,072	$ 1,039