Employee Benefits - Schedule of Fair Value of U.K. Plan Assets (Details) - Pension Plan - U.K. - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total	$ 5,225	$ 5,906	$ 5,675
Cash and cash equivalents
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total	96	209
Pooled funds - Global
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total	209	401
Pooled funds - Europe
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total	3	6
Derivatives
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total	(949)	(771)
Fixed income securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total	2,446	2,787
Annuities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total	1,688	1,909
Pooled funds derivatives
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total	39	57
Pooled funds - Fixed income securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total	850	251
Pooled Funds, Real Estate
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total	149	146
Alternative investments
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total	694	911
Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total	2,175	2,571
Quoted Prices in Active Markets for Identical Assets (Level 1) | Cash and cash equivalents
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total	96	209
Quoted Prices in Active Markets for Identical Assets (Level 1) | Pooled funds - Global
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Pooled funds - Europe
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Derivatives
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fixed income securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total	2,079	2,362
Quoted Prices in Active Markets for Identical Assets (Level 1) | Annuities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Pooled funds derivatives
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Pooled funds - Fixed income securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Pooled Funds, Real Estate
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Alternative investments
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total	0	0
Significant Other Observable Inputs (Level 2)
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total	(582)	(346)
Significant Other Observable Inputs (Level 2) | Cash and cash equivalents
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total	0	0
Significant Other Observable Inputs (Level 2) | Pooled funds - Global
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total	0	0
Significant Other Observable Inputs (Level 2) | Pooled funds - Europe
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total	0	0
Significant Other Observable Inputs (Level 2) | Derivatives
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total	(949)	(771)
Significant Other Observable Inputs (Level 2) | Fixed income securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total	367	425
Significant Other Observable Inputs (Level 2) | Annuities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total	0	0
Significant Other Observable Inputs (Level 2) | Pooled funds derivatives
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total	0
Significant Other Observable Inputs (Level 2) | Pooled funds - Fixed income securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total	0	0
Significant Other Observable Inputs (Level 2) | Pooled Funds, Real Estate
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total	0	0
Significant Other Observable Inputs (Level 2) | Alternative investments
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total	0	0
Significant Unobservable Inputs (Level 3)
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total	1,688	1,909
Significant Unobservable Inputs (Level 3) | Cash and cash equivalents
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total	0	0
Significant Unobservable Inputs (Level 3) | Pooled funds - Global
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total	0	0
Significant Unobservable Inputs (Level 3) | Pooled funds - Europe
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total	0	0
Significant Unobservable Inputs (Level 3) | Derivatives
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total	0	0
Significant Unobservable Inputs (Level 3) | Fixed income securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total	0	0
Significant Unobservable Inputs (Level 3) | Annuities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total	1,688	1,909	$ 1,773
Significant Unobservable Inputs (Level 3) | Pooled funds derivatives
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total	0
Significant Unobservable Inputs (Level 3) | Pooled funds - Fixed income securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total	0	0
Significant Unobservable Inputs (Level 3) | Pooled Funds, Real Estate
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total	0	0
Significant Unobservable Inputs (Level 3) | Alternative investments
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total	$ 0	$ 0