Pension Benefits - Schedule of Fair Value of Plan Assets (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Private Equity
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets		$ 170
Real Estate
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets		1,248
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Funds for retiree health benefits	$ (270)	(282)
Funds for retiree health benefits measured at NAV per share (l)(n)	(37)	(43)
Total funds for retiree health benefits	(307)	325
Investments (excluding funds for retiree health benefits)	12,910	12,223
Pending activities	(438)	(464)
Total fair value of plan net assets	12,472	11,759	$ 11,495	$ 10,755
Pension Benefits | U.S. Equity
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	3,466	3,106
Pension Benefits | International Equity
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	3,558	3,220
Pension Benefits | U.S. Government Issued Debt
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	1,337	2,222
Pension Benefits | Corporate Bonds Debt
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	2,140	1,356
Pension Benefits | Structured Assets Debt
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	1	1
Pension Benefits | Other Fixed Income Debt
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	200	170
Pension Benefits | Cash and Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	536	529
Pension Benefits | Futures
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	364	293
Pension Benefits | Total investments
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	11,602	10,897
Pension Benefits | Private Equity
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	247
Pension Benefits | Real Estate
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	1,139
Pension Benefits | Hedge Funds
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	229	233
Pension Benefits | Investments Valued Using NAV Per Share
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	1,615	1,651
Pension Benefits | Level 1
Defined Benefit Plan Disclosure [Line Items]
Funds for retiree health benefits	(165)	(162)
Investments (excluding funds for retiree health benefits)	6,931	6,094
Pension Benefits | Level 1 | U.S. Equity
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	3,466	3,106
Pension Benefits | Level 1 | International Equity
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	3,187	2,874
Pension Benefits | Level 1 | U.S. Government Issued Debt
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	0	0
Pension Benefits | Level 1 | Corporate Bonds Debt
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	0	0
Pension Benefits | Level 1 | Structured Assets Debt
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	0	0
Pension Benefits | Level 1 | Other Fixed Income Debt
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	0	0
Pension Benefits | Level 1 | Cash and Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	147	115
Pension Benefits | Level 1 | Futures
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	296	161
Pension Benefits | Level 1 | Total investments
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	7,096	6,256
Pension Benefits | Level 2
Defined Benefit Plan Disclosure [Line Items]
Funds for retiree health benefits	(105)	(120)
Investments (excluding funds for retiree health benefits)	4,401	4,521
Pension Benefits | Level 2 | U.S. Equity
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	0
Pension Benefits | Level 2 | International Equity
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	371	346
Pension Benefits | Level 2 | U.S. Government Issued Debt
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	1,337	2,222
Pension Benefits | Level 2 | Corporate Bonds Debt
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	2,140	1,356
Pension Benefits | Level 2 | Structured Assets Debt
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	1	1
Pension Benefits | Level 2 | Other Fixed Income Debt
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	200	170
Pension Benefits | Level 2 | Cash and Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	389	414
Pension Benefits | Level 2 | Futures
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	68	132
Pension Benefits | Level 2 | Total investments
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	$ 4,506	$ 4,641