Employee Retirement Benefits - Fair Value of Pension Plan Assets (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Fair value measurements of plan assets [Abstract]
Fair value	$ 3,563	$ 3,698
Reported Value Measurement [Member]
Fair value measurements of plan assets [Abstract]
Fair value	2,589	2,819
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	1,038	1,173
Significant Other Observable Inputs (Level 2) [Member] | Reported Value Measurement [Member]
Fair value measurements of plan assets [Abstract]
Fair value	1,549	1,646
Significant Unobservable Inputs (Level 3) [Member] | Reported Value Measurement [Member]
Fair value measurements of plan assets [Abstract]
Fair value	2	0
Equity Securities [Member]
Fair value measurements of plan assets [Abstract]
Foreign investments	384	321
Equity Securities [Member] | Reported Value Measurement [Member]
Fair value measurements of plan assets [Abstract]
Fair value	932	1,015
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	910	984
Equity Securities [Member] | Significant Other Observable Inputs (Level 2) [Member] | Reported Value Measurement [Member]
Fair value measurements of plan assets [Abstract]
Fair value	21	31
Equity Securities [Member] | Significant Unobservable Inputs (Level 3) [Member] | Reported Value Measurement [Member]
Fair value measurements of plan assets [Abstract]
Fair value	1	0
Equity Commingled Vehicles [Member]
Fair value measurements of plan assets [Abstract]
Foreign investments	249	306
Equity Commingled Vehicles [Member] | Reported Value Measurement [Member]
Fair value measurements of plan assets [Abstract]
Fair value	792	767
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	792	767
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
Us Government And Municipal Bonds [Member] | Reported Value Measurement [Member]
Fair value measurements of plan assets [Abstract]
Fair value	123	164
Us 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	110	144
Us 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	20
Us 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	68	88
Corporate debt securities [Member] | Reported Value Measurement [Member]
Fair value measurements of plan assets [Abstract]
Fair value	280	355
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	2	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	277	355
Corporate debt securities [Member] | Significant Unobservable Inputs (Level 3) [Member] | Reported Value Measurement [Member]
Fair value measurements of plan assets [Abstract]
Fair value	1	0
Asset-backed securities [Member] | Reported Value Measurement [Member]
Fair value measurements of plan assets [Abstract]
Fair value	167	223
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	167	223
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] | Reported Value Measurement [Member]
Fair value measurements of plan assets [Abstract]
Fair value	21	21
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	21	21
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	23
Convertible securities [Member] | Reported Value Measurement [Member]
Fair value measurements of plan assets [Abstract]
Fair value	274	274
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	16	45
Convertible securities [Member] | Significant Other Observable Inputs (Level 2) [Member] | Reported Value Measurement [Member]
Fair value measurements of plan assets [Abstract]
Fair value	258	229
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	283	200
Investments measured at net asset value [Member] | Reported Value Measurement [Member]
Fair value measurements of plan assets [Abstract]
Fair value	$ 974	$ 879