Pension Benefits - Schedule of Fair Value of Plan Assets (Detail) (Pension Benefits [Member], USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Defined Benefit Plan Disclosure [Line Items]
Total investments	$ 12,415	$ 11,759
Funds for retiree health benefits	(358)	(372)
Investments (excluding funds for retiree health benefits)	12,057	11,387
Pending activities	(562)	(632)
Total fair value of plan net assets	11,495	10,755	9,135	7,800
U.S. Equity [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments	3,168	3,057
International Equity [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments	3,202	3,174
Private Equity [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments	114	67
Total fair value of plan net assets	114	67	20
U.S. Government Issued Debt [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments	2,113	1,855
Corporate Bonds Debt [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments	1,351	1,151
Structured Assets Debt [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments	4	4
Other Fixed Income Debt [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments	208	150
Real Estate [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments	1,137	1,062
Total fair value of plan net assets	1,137	1,062	833
Cash and Cash Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments	665	685
Futures [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments	229	348
Hedge Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments	224	206
Total fair value of plan net assets	224	206
Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments	6,389	5,835
Funds for retiree health benefits	(184)	(185)
Investments (excluding funds for retiree health benefits)	6,205	5,650
Level 1 [Member] | U.S. Equity [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments	3,168	3,057
Level 1 [Member] | International Equity [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments	2,841	2,303
Level 1 [Member] | Cash and Cash Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments	188	127
Level 1 [Member] | Futures [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments	192	348
Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments	4,551	4,589
Funds for retiree health benefits	(131)	(145)
Investments (excluding funds for retiree health benefits)	4,420	4,444
Level 2 [Member] | International Equity [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments	361	871
Level 2 [Member] | U.S. Government Issued Debt [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments	2,113	1,855
Level 2 [Member] | Corporate Bonds Debt [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments	1,351	1,151
Level 2 [Member] | Structured Assets Debt [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments	4	4
Level 2 [Member] | Other Fixed Income Debt [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments	208	150
Level 2 [Member] | Cash and Cash Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments	477	558
Level 2 [Member] | Futures [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments	37
Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments	1,475	1,335
Funds for retiree health benefits	(43)	(42)
Investments (excluding funds for retiree health benefits)	1,432	1,293
Level 3 [Member] | Private Equity [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments	114	67
Level 3 [Member] | Real Estate [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments	1,137	1,062
Level 3 [Member] | Hedge Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments	$ 224	$ 206