Pension Benefits - Schedule of Fair Value of Plan Assets (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	$ 330	$ 273
Private Equity
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets		247
Real Estate
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets		1,139
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Funds for retiree health benefits	(262)	(270)
Funds for retiree health benefits measured at NAV per share (l)(n)	(36)	(37)
Total funds for retiree health benefits	(298)	307
Investments (excluding funds for retiree health benefits)	14,683	12,910
Pending activities	(409)	(438)
Total fair value of plan net assets	14,274	12,472	$ 11,759	$ 11,495
Pension Benefits | U.S. Equity
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	3,900	3,466
Pension Benefits | International Equity
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	4,132	3,558
Pension Benefits | U.S. Government Issued Debt
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	1,786	1,337
Pension Benefits | Corporate Bonds Debt
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	2,450	2,140
Pension Benefits | Structured Assets Debt
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	3	1
Pension Benefits | Other Fixed Income Debt
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	125	200
Pension Benefits | Cash and Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	476	536
Pension Benefits | Futures
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	308	364
Pension Benefits | Total investments
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	13,180	11,602
Pension Benefits | Private Equity
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	336
Pension Benefits | Real Estate
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	1,214
Pension Benefits | Hedge Funds
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	251	229
Pension Benefits | Investments Valued Using NAV Per Share
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	1,801	1,615
Pension Benefits | Level 1
Defined Benefit Plan Disclosure [Line Items]
Funds for retiree health benefits	(168)	(165)
Investments (excluding funds for retiree health benefits)	8,268	6,931
Pension Benefits | Level 1 | U.S. Equity
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	3,872	3,466
Pension Benefits | Level 1 | International Equity
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	4,132	3,187
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	124	147
Pension Benefits | Level 1 | Futures
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	308	296
Pension Benefits | Level 1 | Total investments
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	8,436	7,096
Pension Benefits | Level 2
Defined Benefit Plan Disclosure [Line Items]
Funds for retiree health benefits	(94)	(105)
Investments (excluding funds for retiree health benefits)	4,650	4,401
Pension Benefits | Level 2 | U.S. Equity
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	28
Pension Benefits | Level 2 | International Equity
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets		371
Pension Benefits | Level 2 | U.S. Government Issued Debt
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	1,786	1,337
Pension Benefits | Level 2 | Corporate Bonds Debt
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	2,450	2,140
Pension Benefits | Level 2 | Structured Assets Debt
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	3	1
Pension Benefits | Level 2 | Other Fixed Income Debt
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	125	200
Pension Benefits | Level 2 | Cash and Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	352	389
Pension Benefits | Level 2 | Futures
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets		68
Pension Benefits | Level 2 | Total investments
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	$ 4,744	$ 4,506