Employee Benefits - Schedule of Fair Value of U.S Plan Assets (Details) - Pension Plan - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
U.S.
Defined Benefit Plan Disclosure [Line Items]
Total	$ 1,796	$ 1,958	$ 1,683
U.S. | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Total	130	56
U.S. | Large cap domestic
Defined Benefit Plan Disclosure [Line Items]
Total	294	313
U.S. | Small cap domestic
Defined Benefit Plan Disclosure [Line Items]
Total	14	17
U.S. | International
Defined Benefit Plan Disclosure [Line Items]
Total	76	90
U.S. | Equity derivatives
Defined Benefit Plan Disclosure [Line Items]
Total	(14)	111
U.S. | International pooled funds
Defined Benefit Plan Disclosure [Line Items]
Total	235	270
U.S. | Pooled funds large cap domestic
Defined Benefit Plan Disclosure [Line Items]
Total	8	12
U.S. | Pooled funds small cap domestic
Defined Benefit Plan Disclosure [Line Items]
Total	42	114
U.S. | Corporate bonds
Defined Benefit Plan Disclosure [Line Items]
Total	111	110
U.S. | Government and agency bonds
Defined Benefit Plan Disclosure [Line Items]
Total	126	148
U.S. | Asset-backed securities
Defined Benefit Plan Disclosure [Line Items]
Total	2
U.S. | Government and agency bonds
Defined Benefit Plan Disclosure [Line Items]
Total	95
U.S. | Corporate Bonds And Pooled Funds
Defined Benefit Plan Disclosure [Line Items]
Total	322	290
U.S. | Real estate and REITS
Defined Benefit Plan Disclosure [Line Items]
Total	78	82
U.S. | Alternative investments
Defined Benefit Plan Disclosure [Line Items]
Total	277	345
U.S. | Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Total	687	672
U.S. | Quoted Prices in Active Markets for Identical Assets (Level 1) | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Total	130	56
U.S. | Quoted Prices in Active Markets for Identical Assets (Level 1) | Large cap domestic
Defined Benefit Plan Disclosure [Line Items]
Total	294	313
U.S. | Quoted Prices in Active Markets for Identical Assets (Level 1) | Small cap domestic
Defined Benefit Plan Disclosure [Line Items]
Total	14	17
U.S. | Quoted Prices in Active Markets for Identical Assets (Level 1) | International
Defined Benefit Plan Disclosure [Line Items]
Total	76	90
U.S. | Quoted Prices in Active Markets for Identical Assets (Level 1) | Equity derivatives
Defined Benefit Plan Disclosure [Line Items]
Total	0	0
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	95	114
U.S. | Quoted Prices in Active Markets for Identical Assets (Level 1) | Asset-backed securities
Defined Benefit Plan Disclosure [Line Items]
Total	0
U.S. | Quoted Prices in Active Markets for Identical Assets (Level 1) | Government and agency bonds
Defined Benefit Plan Disclosure [Line Items]
Total	0
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) | Real estate and REITS
Defined Benefit Plan Disclosure [Line Items]
Total	78	82
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	130	255
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) | 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	(14)	111
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	111	110
U.S. | Significant Other Observable Inputs (Level 2) | Government and agency bonds
Defined Benefit Plan Disclosure [Line Items]
Total	31	34
U.S. | Significant Other Observable Inputs (Level 2) | Asset-backed securities
Defined Benefit Plan Disclosure [Line Items]
Total	2
U.S. | Significant Other Observable Inputs (Level 2) | Government and agency bonds
Defined Benefit Plan Disclosure [Line Items]
Total	0
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) | 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) | 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	0
U.S. | Significant Unobservable Inputs (Level 3) | International
Defined Benefit Plan Disclosure [Line Items]
Total	0	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) | Asset-backed securities
Defined Benefit Plan Disclosure [Line Items]
Total	0
U.S. | Significant Unobservable Inputs (Level 3) | Government and agency bonds
Defined Benefit Plan Disclosure [Line Items]
Total	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) | 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
U.K.
Defined Benefit Plan Disclosure [Line Items]
Total	5,225	5,906	$ 5,675
U.K. | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Total	96	209
U.K. | Alternative investments
Defined Benefit Plan Disclosure [Line Items]
Total	694	911
U.K. | Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Total	2,175	2,571
U.K. | Quoted Prices in Active Markets for Identical Assets (Level 1) | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Total	96	209
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	(582)	(346)
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) | 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	1,688	1,909
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) | Alternative investments
Defined Benefit Plan Disclosure [Line Items]
Total	$ 0	$ 0