Pension Benefits - Schedule of Fair Value of Plan Assets (Detail) - Pension Benefits - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Defined Benefit Plan Disclosure [Line Items]
Total investments	$ 12,548	$ 12,415
Funds for retiree health benefits	(325)	(358)
Investments (excluding funds for retiree health benefits)	12,223	12,057
Pending activities	(464)	(562)
Total fair value of plan net assets	11,759	11,495	$ 10,755	$ 9,135
U.S. Equity
Defined Benefit Plan Disclosure [Line Items]
Total investments	3,106	3,168
International Equity
Defined Benefit Plan Disclosure [Line Items]
Total investments	3,220	3,202
Private Equity
Defined Benefit Plan Disclosure [Line Items]
Total investments	170	114
Total fair value of plan net assets	170	114
U.S. Government Issued Debt
Defined Benefit Plan Disclosure [Line Items]
Total investments	2,222	2,113
Corporate Bonds Debt
Defined Benefit Plan Disclosure [Line Items]
Total investments	1,356	1,351
Structured Assets Debt
Defined Benefit Plan Disclosure [Line Items]
Total investments	1	4
Other Fixed Income Debt
Defined Benefit Plan Disclosure [Line Items]
Total investments	170	208
Real Estate
Defined Benefit Plan Disclosure [Line Items]
Total investments	1,248	1,137
Total fair value of plan net assets	1,248	1,137
Cash and Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Total investments	529	665
Futures
Defined Benefit Plan Disclosure [Line Items]
Total investments	293	229
Hedge Funds
Defined Benefit Plan Disclosure [Line Items]
Total investments	233	224
Total fair value of plan net assets	233	224
Level 1
Defined Benefit Plan Disclosure [Line Items]
Total investments	6,256	6,389
Funds for retiree health benefits	(162)	(184)
Investments (excluding funds for retiree health benefits)	6,094	6,205
Level 1 | U.S. Equity
Defined Benefit Plan Disclosure [Line Items]
Total investments	3,106	3,168
Level 1 | International Equity
Defined Benefit Plan Disclosure [Line Items]
Total investments	2,874	2,841
Level 1 | Cash and Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Total investments	115	188
Level 1 | Futures
Defined Benefit Plan Disclosure [Line Items]
Total investments	161	192
Level 2
Defined Benefit Plan Disclosure [Line Items]
Total investments	4,641	4,551
Funds for retiree health benefits	(120)	(131)
Investments (excluding funds for retiree health benefits)	4,521	4,420
Level 2 | International Equity
Defined Benefit Plan Disclosure [Line Items]
Total investments	346	361
Level 2 | U.S. Government Issued Debt
Defined Benefit Plan Disclosure [Line Items]
Total investments	2,222	2,113
Level 2 | Corporate Bonds Debt
Defined Benefit Plan Disclosure [Line Items]
Total investments	1,356	1,351
Level 2 | Structured Assets Debt
Defined Benefit Plan Disclosure [Line Items]
Total investments	1	4
Level 2 | Other Fixed Income Debt
Defined Benefit Plan Disclosure [Line Items]
Total investments	170	208
Level 2 | Cash and Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Total investments	414	477
Level 2 | Futures
Defined Benefit Plan Disclosure [Line Items]
Total investments	132	37
Level 3
Defined Benefit Plan Disclosure [Line Items]
Total investments	1,651	1,475
Funds for retiree health benefits	(43)	(43)
Investments (excluding funds for retiree health benefits)	1,608	1,432
Level 3 | Private Equity
Defined Benefit Plan Disclosure [Line Items]
Total investments	170	114
Total fair value of plan net assets		114	67
Level 3 | Real Estate
Defined Benefit Plan Disclosure [Line Items]
Total investments	1,248	1,137
Total fair value of plan net assets		1,137	1,062
Level 3 | Hedge Funds
Defined Benefit Plan Disclosure [Line Items]
Total investments	$ 233	224
Total fair value of plan net assets		$ 224	$ 206