Employee Retirement Benefits - Fair Value of Pension Plan Assets (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Fair value measurements of plan assets [Abstract]
Fair value	$ 4,020	$ 3,651
Reported Value Measurement [Member]
Fair value measurements of plan assets [Abstract]
Fair value	2,981	2,592
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	1,217	1,046
Significant Other Observable Inputs (Level 2) [Member] | Reported Value Measurement [Member]
Fair value measurements of plan assets [Abstract]
Fair value	1,752	1,542
Significant Unobservable Inputs (Level 3) [Member] | Reported Value Measurement [Member]
Fair value measurements of plan assets [Abstract]
Fair value	12	4
Equity Securities [Member]
Fair value measurements of plan assets [Abstract]
Foreign investments	480	370
Equity Securities [Member] | Reported Value Measurement [Member]
Fair value measurements of plan assets [Abstract]
Fair value	1,095	898
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	1,077	879
Equity Securities [Member] | Significant Other Observable Inputs (Level 2) [Member] | Reported Value Measurement [Member]
Fair value measurements of plan assets [Abstract]
Fair value	16	16
Equity Securities [Member] | Significant Unobservable Inputs (Level 3) [Member] | Reported Value Measurement [Member]
Fair value measurements of plan assets [Abstract]
Fair value	2	3
Equity Commingled Vehicles [Member]
Fair value measurements of plan assets [Abstract]
Foreign investments	287	261
Equity Commingled Vehicles [Member] | Reported Value Measurement [Member]
Fair value measurements of plan assets [Abstract]
Fair value	853	845
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	853	845
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	131	155
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	118	143
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	13	12
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	73	67
Corporate debt securities [Member] | Reported Value Measurement [Member]
Fair value measurements of plan assets [Abstract]
Fair value	251	250
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	3	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	238	246
Corporate debt securities [Member] | Significant Unobservable Inputs (Level 3) [Member] | Reported Value Measurement [Member]
Fair value measurements of plan assets [Abstract]
Fair value	10	1
Asset-backed securities [Member] | Reported Value Measurement [Member]
Fair value measurements of plan assets [Abstract]
Fair value	170	124
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	170	124
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	155	22
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	155	22
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	31
Convertible securities [Member] | Reported Value Measurement [Member]
Fair value measurements of plan assets [Abstract]
Fair value	326	298
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	19	21
Convertible securities [Member] | Significant Other Observable Inputs (Level 2) [Member] | Reported Value Measurement [Member]
Fair value measurements of plan assets [Abstract]
Fair value	307	277
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	233	282
Investments measured at net asset value [Member] | Reported Value Measurement [Member]
Fair value measurements of plan assets [Abstract]
Fair value	$ 1,039	$ 1,059