Employee Benefits - Schedule of Fair Value of Other Plan Assets (Details) - Other - Pension Plan - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total	$ 1,256	$ 1,076	$ 1,019
Cash and cash equivalents
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total	11	11
Pooled funds - Global
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total	370	322
Pooled funds small cap domestic
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total	26	36
Derivatives
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total		20
Fixed income securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total	211	166
Derivatives
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total	40	37
Pooled funds - Fixed income securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total	566	469
Alternative investments
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total	26	9
Pooled Funds, Real Estate
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total	6	6
Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total	11	11
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	11	11
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 small cap domestic
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
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	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) | 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) | Alternative investments
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
Significant Other Observable Inputs (Level 2)
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total	251	203
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 small cap domestic
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		0
Significant Other Observable Inputs (Level 2) | Fixed income securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total	211	166
Significant Other Observable Inputs (Level 2) | Derivatives
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total	40	37
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) | Alternative investments
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 Unobservable Inputs (Level 3)
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total	0	0
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 small cap domestic
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
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) | Derivatives
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total	0	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) | Alternative investments
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