Benefit Plans - Schedule of Fair Value of Pension Plan Assets by Three Level Fair Value Hierarchy (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Total	$ 6,152	$ 4,824
Level 1
Defined Benefit Plan Disclosure [Line Items]
Total	1,605	1,372
Level 2
Defined Benefit Plan Disclosure [Line Items]
Total	4,547	3,452
Cash and cash-equivalents
Defined Benefit Plan Disclosure [Line Items]
Total	67	179
Cash and cash-equivalents | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total	67	179
Cash and cash-equivalents | Level 2
Defined Benefit Plan Disclosure [Line Items]
Total	0	0
U.S. equity securities
Defined Benefit Plan Disclosure [Line Items]
Total	2,503	1,892
U.S. equity securities | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total	1,333	1,018
U.S. equity securities | Level 2
Defined Benefit Plan Disclosure [Line Items]
Total	1,170	874
International equity securities
Defined Benefit Plan Disclosure [Line Items]
Total	1,130	839
International equity securities | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total	195	165
International equity securities | Level 2
Defined Benefit Plan Disclosure [Line Items]
Total	935	674
Fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Total	2,452	1,914
Fixed income securities | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total	10	10
Fixed income securities | Level 2
Defined Benefit Plan Disclosure [Line Items]
Total	$ 2,442	$ 1,904