Employee Benefits - Schedule of Fair Value of U.S Plan Assets (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
U.K. [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	$ 5,903	$ 6,224	$ 5,398
U.K. [Member] | Pooled Funds, Real Estate [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	65	39
U.K. [Member] | Cash and cash equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	159	224
U.K. [Member] | Equity derivatives [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	66	0
U.K. [Member] | Alternative investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	717	968
U.K. [Member] | Real Estate, Excluding Pooled Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	20	7
U.K. [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Total	2,621	3,643
U.K. [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pooled Funds, Real Estate [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	0	0
U.K. [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Cash and cash equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	159	224
U.K. [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Equity derivatives [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	0	0
U.K. [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Alternative investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	0	0
U.K. [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Real Estate, Excluding Pooled Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Total		0
U.K. [Member] | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Total	1,633	731
U.K. [Member] | Significant Other Observable Inputs (Level 2) | Pooled Funds, Real Estate [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	0	0
U.K. [Member] | Significant Other Observable Inputs (Level 2) | Cash and cash equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	0	0
U.K. [Member] | Significant Other Observable Inputs (Level 2) | Equity derivatives [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	66	0
U.K. [Member] | Significant Other Observable Inputs (Level 2) | Alternative investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	4	0
U.K. [Member] | Significant Other Observable Inputs (Level 2) | Real Estate, Excluding Pooled Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Total		0
U.K. [Member] | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Total	1,649	1,850	1,133
U.K. [Member] | Significant Unobservable Inputs (Level 3) | Pooled Funds, Real Estate [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	65	39
U.K. [Member] | Significant Unobservable Inputs (Level 3) | Cash and cash equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	0	0
U.K. [Member] | Significant Unobservable Inputs (Level 3) | Equity derivatives [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	0	0
U.K. [Member] | Significant Unobservable Inputs (Level 3) | Alternative investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	713	968	546
U.K. [Member] | Significant Unobservable Inputs (Level 3) | Real Estate, Excluding Pooled Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	20	7
U.S. Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	1,951	2,036	1,855
U.S. Pension Plan [Member] | Cash and cash equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	33	68
U.S. Pension Plan [Member] | Large cap domestic
Defined Benefit Plan Disclosure [Line Items]
Total	299	329
U.S. Pension Plan [Member] | Small cap domestic [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	88	85
U.S. Pension Plan [Member] | Large cap international [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	262	258
U.S. Pension Plan [Member] | Equity derivatives [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	203	285
U.S. Pension Plan [Member] | Corporate bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	484	503
U.S. Pension Plan [Member] | Government and agency bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	128	109
U.S. Pension Plan [Member] | Asset-backed securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	0	20
U.S. Pension Plan [Member] | Fixed income derivatives [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	69	49
U.S. Pension Plan [Member] | Alternative investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	305	272
U.S. Pension Plan [Member] | Commodity derivatives [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	13	(8)
U.S. Pension Plan [Member] | Real estate and REITS [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	67	66
U.S. Pension Plan [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Total	750	837
U.S. Pension Plan [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Cash and cash equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	33	68
U.S. Pension Plan [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Large cap domestic
Defined Benefit Plan Disclosure [Line Items]
Total	299	329
U.S. Pension Plan [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Small cap domestic [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	30	22
U.S. Pension Plan [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Large cap international [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	52	114
U.S. Pension Plan [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Equity derivatives [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	170	209
U.S. Pension Plan [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Corporate bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	0	0
U.S. Pension Plan [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Government and agency bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	52	29
U.S. Pension Plan [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Asset-backed securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	0	0
U.S. Pension Plan [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fixed income derivatives [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	47	0
U.S. Pension Plan [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Alternative investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	0	0
U.S. Pension Plan [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Commodity derivatives [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	0	0
U.S. Pension Plan [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Real estate and REITS [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	67	66
U.S. Pension Plan [Member] | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Total	560	575
U.S. Pension Plan [Member] | Significant Other Observable Inputs (Level 2) | Cash and cash equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	0	0
U.S. Pension Plan [Member] | Significant Other Observable Inputs (Level 2) | Large cap domestic
Defined Benefit Plan Disclosure [Line Items]
Total	0	0
U.S. Pension Plan [Member] | Significant Other Observable Inputs (Level 2) | Small cap domestic [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	58	63
U.S. Pension Plan [Member] | Significant Other Observable Inputs (Level 2) | Large cap international [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	210	144
U.S. Pension Plan [Member] | Significant Other Observable Inputs (Level 2) | Equity derivatives [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	33	76
U.S. Pension Plan [Member] | Significant Other Observable Inputs (Level 2) | Corporate bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	148	151
U.S. Pension Plan [Member] | Significant Other Observable Inputs (Level 2) | Government and agency bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	76	80
U.S. Pension Plan [Member] | Significant Other Observable Inputs (Level 2) | Asset-backed securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	0	20
U.S. Pension Plan [Member] | Significant Other Observable Inputs (Level 2) | Fixed income derivatives [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	22	49
U.S. Pension Plan [Member] | Significant Other Observable Inputs (Level 2) | Alternative investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	0	0
U.S. Pension Plan [Member] | Significant Other Observable Inputs (Level 2) | Commodity derivatives [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	13	(8)
U.S. Pension Plan [Member] | Significant Other Observable Inputs (Level 2) | Real estate and REITS [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	0	0
U.S. Pension Plan [Member] | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Total	641	624	$ 266
U.S. Pension Plan [Member] | Significant Unobservable Inputs (Level 3) | Cash and cash equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	0	0
U.S. Pension Plan [Member] | Significant Unobservable Inputs (Level 3) | Large cap domestic
Defined Benefit Plan Disclosure [Line Items]
Total	0	0
U.S. Pension Plan [Member] | Significant Unobservable Inputs (Level 3) | Small cap domestic [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	0	0
U.S. Pension Plan [Member] | Significant Unobservable Inputs (Level 3) | Large cap international [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	0	0
U.S. Pension Plan [Member] | Significant Unobservable Inputs (Level 3) | Equity derivatives [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	0	0
U.S. Pension Plan [Member] | Significant Unobservable Inputs (Level 3) | Corporate bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	336	352
U.S. Pension Plan [Member] | Significant Unobservable Inputs (Level 3) | Government and agency bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	0	0
U.S. Pension Plan [Member] | Significant Unobservable Inputs (Level 3) | Asset-backed securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	0	0
U.S. Pension Plan [Member] | Significant Unobservable Inputs (Level 3) | Fixed income derivatives [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	0	0
U.S. Pension Plan [Member] | Significant Unobservable Inputs (Level 3) | Alternative investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	305	272
U.S. Pension Plan [Member] | Significant Unobservable Inputs (Level 3) | Commodity derivatives [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	0	0
U.S. Pension Plan [Member] | Significant Unobservable Inputs (Level 3) | Real estate and REITS [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	$ 0	$ 0