Employee Benefits - Schedule of Fair Value of U.S Plan Assets (Details) - Pension Plan - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
U.K.
Defined Benefit Plan Disclosure [Line Items]
Total	$ 5,906	$ 5,675	$ 5,903
U.K. | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Total	209	86
U.K. | Equity derivatives
Defined Benefit Plan Disclosure [Line Items]
Total	(771)	10
U.K. | Alternative investments
Defined Benefit Plan Disclosure [Line Items]
Total	911	773
U.K. | Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Total	2,571	1,947
U.K. | Quoted Prices in Active Markets for Identical Assets (Level 1) | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Total	209	86
U.K. | Quoted Prices in Active Markets for Identical Assets (Level 1) | Equity derivatives
Defined Benefit Plan Disclosure [Line Items]
Total	0	0
U.K. | Quoted Prices in Active Markets for Identical Assets (Level 1) | Alternative investments
Defined Benefit Plan Disclosure [Line Items]
Total	0	0
U.K. | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Total	(346)	413
U.K. | Significant Other Observable Inputs (Level 2) | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Total	0	0
U.K. | Significant Other Observable Inputs (Level 2) | Equity derivatives
Defined Benefit Plan Disclosure [Line Items]
Total	(771)	10
U.K. | Significant Other Observable Inputs (Level 2) | Alternative investments
Defined Benefit Plan Disclosure [Line Items]
Total	0	0
U.K. | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Total	1,909	1,773
U.K. | Significant Unobservable Inputs (Level 3) | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Total	0	0
U.K. | Significant Unobservable Inputs (Level 3) | Equity derivatives
Defined Benefit Plan Disclosure [Line Items]
Total	0	0
U.K. | Significant Unobservable Inputs (Level 3) | Alternative investments
Defined Benefit Plan Disclosure [Line Items]
Total	0	0
U.S.
Defined Benefit Plan Disclosure [Line Items]
Total	1,958	1,683	$ 1,951
U.S. | Fixed income derivatives
Defined Benefit Plan Disclosure [Line Items]
Total		65
U.S. | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Total	56	100
U.S. | Large cap domestic
Defined Benefit Plan Disclosure [Line Items]
Total	313	268
U.S. | Small cap domestic
Defined Benefit Plan Disclosure [Line Items]
Total	17	15
U.S. | Large cap international
Defined Benefit Plan Disclosure [Line Items]
Total	90	64
U.S. | Equity derivatives
Defined Benefit Plan Disclosure [Line Items]
Total	111	81
U.S. | International pooled funds
Defined Benefit Plan Disclosure [Line Items]
Total	270	196
U.S. | Pooled funds large cap domestic
Defined Benefit Plan Disclosure [Line Items]
Total	12
U.S. | Pooled funds small cap domestic
Defined Benefit Plan Disclosure [Line Items]
Total	114	52
U.S. | Corporate bonds
Defined Benefit Plan Disclosure [Line Items]
Total	110	105
U.S. | Government and agency bonds
Defined Benefit Plan Disclosure [Line Items]
Total	148	132
U.S. | Corporate Bonds And Pooled Funds
Defined Benefit Plan Disclosure [Line Items]
Total	290	255
U.S. | Commodity Derivatives
Defined Benefit Plan Disclosure [Line Items]
Total		22
U.S. | Real estate and REITS
Defined Benefit Plan Disclosure [Line Items]
Total	82	61
U.S. | Alternative investments
Defined Benefit Plan Disclosure [Line Items]
Total	345	267
U.S. | Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Total	672	727
U.S. | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fixed income derivatives
Defined Benefit Plan Disclosure [Line Items]
Total		65
U.S. | Quoted Prices in Active Markets for Identical Assets (Level 1) | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Total	56	100
U.S. | Quoted Prices in Active Markets for Identical Assets (Level 1) | Large cap domestic
Defined Benefit Plan Disclosure [Line Items]
Total	313	268
U.S. | Quoted Prices in Active Markets for Identical Assets (Level 1) | Small cap domestic
Defined Benefit Plan Disclosure [Line Items]
Total	17	15
U.S. | Quoted Prices in Active Markets for Identical Assets (Level 1) | Large cap international
Defined Benefit Plan Disclosure [Line Items]
Total	90	64
U.S. | Quoted Prices in Active Markets for Identical Assets (Level 1) | Equity derivatives
Defined Benefit Plan Disclosure [Line Items]
Total	0	78
U.S. | Quoted Prices in Active Markets for Identical Assets (Level 1) | International pooled funds
Defined Benefit Plan Disclosure [Line Items]
Total	0	0
U.S. | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pooled funds large cap domestic
Defined Benefit Plan Disclosure [Line Items]
Total	0
U.S. | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pooled funds small cap domestic
Defined Benefit Plan Disclosure [Line Items]
Total	0	0
U.S. | Quoted Prices in Active Markets for Identical Assets (Level 1) | Corporate bonds
Defined Benefit Plan Disclosure [Line Items]
Total	0	0
U.S. | Quoted Prices in Active Markets for Identical Assets (Level 1) | Government and agency bonds
Defined Benefit Plan Disclosure [Line Items]
Total	114	76
U.S. | Quoted Prices in Active Markets for Identical Assets (Level 1) | Corporate Bonds And Pooled Funds
Defined Benefit Plan Disclosure [Line Items]
Total	0	0
U.S. | Quoted Prices in Active Markets for Identical Assets (Level 1) | Commodity Derivatives
Defined Benefit Plan Disclosure [Line Items]
Total		0
U.S. | Quoted Prices in Active Markets for Identical Assets (Level 1) | Real estate and REITS
Defined Benefit Plan Disclosure [Line Items]
Total	82	61
U.S. | Quoted Prices in Active Markets for Identical Assets (Level 1) | Alternative investments
Defined Benefit Plan Disclosure [Line Items]
Total	0	0
U.S. | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Total	255	186
U.S. | Significant Other Observable Inputs (Level 2) | Fixed income derivatives
Defined Benefit Plan Disclosure [Line Items]
Total		0
U.S. | Significant Other Observable Inputs (Level 2) | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Total	0	0
U.S. | Significant Other Observable Inputs (Level 2) | Large cap domestic
Defined Benefit Plan Disclosure [Line Items]
Total	0	0
U.S. | Significant Other Observable Inputs (Level 2) | Small cap domestic
Defined Benefit Plan Disclosure [Line Items]
Total	0	0
U.S. | Significant Other Observable Inputs (Level 2) | Large cap international
Defined Benefit Plan Disclosure [Line Items]
Total	0	0
U.S. | Significant Other Observable Inputs (Level 2) | Equity derivatives
Defined Benefit Plan Disclosure [Line Items]
Total	111	3
U.S. | Significant Other Observable Inputs (Level 2) | International pooled funds
Defined Benefit Plan Disclosure [Line Items]
Total	0	0
U.S. | Significant Other Observable Inputs (Level 2) | Pooled funds large cap domestic
Defined Benefit Plan Disclosure [Line Items]
Total	0
U.S. | Significant Other Observable Inputs (Level 2) | Pooled funds small cap domestic
Defined Benefit Plan Disclosure [Line Items]
Total	0	0
U.S. | Significant Other Observable Inputs (Level 2) | Corporate bonds
Defined Benefit Plan Disclosure [Line Items]
Total	110	105
U.S. | Significant Other Observable Inputs (Level 2) | Government and agency bonds
Defined Benefit Plan Disclosure [Line Items]
Total	34	56
U.S. | Significant Other Observable Inputs (Level 2) | Corporate Bonds And Pooled Funds
Defined Benefit Plan Disclosure [Line Items]
Total	0	0
U.S. | Significant Other Observable Inputs (Level 2) | Commodity Derivatives
Defined Benefit Plan Disclosure [Line Items]
Total		22
U.S. | Significant Other Observable Inputs (Level 2) | Real estate and REITS
Defined Benefit Plan Disclosure [Line Items]
Total	0	0
U.S. | Significant Other Observable Inputs (Level 2) | Alternative investments
Defined Benefit Plan Disclosure [Line Items]
Total	0	0
U.S. | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Total	0	0
U.S. | Significant Unobservable Inputs (Level 3) | Fixed income derivatives
Defined Benefit Plan Disclosure [Line Items]
Total		0
U.S. | Significant Unobservable Inputs (Level 3) | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Total	0	0
U.S. | Significant Unobservable Inputs (Level 3) | Large cap domestic
Defined Benefit Plan Disclosure [Line Items]
Total	0	0
U.S. | Significant Unobservable Inputs (Level 3) | Small cap domestic
Defined Benefit Plan Disclosure [Line Items]
Total		0
U.S. | Significant Unobservable Inputs (Level 3) | Large cap international
Defined Benefit Plan Disclosure [Line Items]
Total		0
U.S. | Significant Unobservable Inputs (Level 3) | Equity derivatives
Defined Benefit Plan Disclosure [Line Items]
Total	0	0
U.S. | Significant Unobservable Inputs (Level 3) | International pooled funds
Defined Benefit Plan Disclosure [Line Items]
Total	0	0
U.S. | Significant Unobservable Inputs (Level 3) | Pooled funds large cap domestic
Defined Benefit Plan Disclosure [Line Items]
Total	0
U.S. | Significant Unobservable Inputs (Level 3) | Pooled funds small cap domestic
Defined Benefit Plan Disclosure [Line Items]
Total	0	0
U.S. | Significant Unobservable Inputs (Level 3) | Corporate bonds
Defined Benefit Plan Disclosure [Line Items]
Total	0	0
U.S. | Significant Unobservable Inputs (Level 3) | Government and agency bonds
Defined Benefit Plan Disclosure [Line Items]
Total	0	0
U.S. | Significant Unobservable Inputs (Level 3) | Corporate Bonds And Pooled Funds
Defined Benefit Plan Disclosure [Line Items]
Total	0	0
U.S. | Significant Unobservable Inputs (Level 3) | Commodity Derivatives
Defined Benefit Plan Disclosure [Line Items]
Total		0
U.S. | Significant Unobservable Inputs (Level 3) | Real estate and REITS
Defined Benefit Plan Disclosure [Line Items]
Total	0	0
U.S. | Significant Unobservable Inputs (Level 3) | Alternative investments
Defined Benefit Plan Disclosure [Line Items]
Total	$ 0	$ 0