Pension Benefits - Schedule of Fair Value of Plan Assets (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Private Equity [Member]
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	$ 67	$ 20
Real Estate [Member]
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	1,062	833
Hedge Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	206
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments	11,759	9,878
Funds for retiree health benefits	(372)	(353)
Investments (excluding funds for retiree health benefits)	11,387	9,525
Pending activities	(632)	(390)
Total fair value of plan net assets	10,755	9,135	7,800	7,721
Pension Benefits [Member] | U.S. Equity [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments	3,057	2,637
Pension Benefits [Member] | International Equity [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments	3,174	2,995
Pension Benefits [Member] | Private Equity [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments	67	20
Total fair value of plan net assets		20
Pension Benefits [Member] | U.S. Government Issued Debt [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments	1,855	1,626
Pension Benefits [Member] | Corporate Bonds Debt [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments	1,151	993
Total fair value of plan net assets			94
Pension Benefits [Member] | Structured Assets Debt [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments	4	30
Total fair value of plan net assets			13
Pension Benefits [Member] | Other Fixed Income Debt [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments	150	123
Total fair value of plan net assets			29
Pension Benefits [Member] | Real Estate [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments	1,062	833
Total fair value of plan net assets		833	572
Pension Benefits [Member] | Cash and Cash Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments	685	411
Pension Benefits [Member] | Futures [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments	348	210
Pension Benefits [Member] | Hedge Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments	206
Pension Benefits [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments	5,835	5,172
Funds for retiree health benefits	(185)	(185)
Investments (excluding funds for retiree health benefits)	5,650	4,987
Pension Benefits [Member] | Level 1 [Member] | U.S. Equity [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments	3,057	2,637
Pension Benefits [Member] | Level 1 [Member] | International Equity [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments	2,303	2,242
Pension Benefits [Member] | Level 1 [Member] | Cash and Cash Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments	127	83
Pension Benefits [Member] | Level 1 [Member] | Futures [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments	348	210
Pension Benefits [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments	4,589	3,853
Funds for retiree health benefits	(145)	(137)
Investments (excluding funds for retiree health benefits)	4,444	3,716
Pension Benefits [Member] | Level 2 [Member] | International Equity [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments	871	753
Pension Benefits [Member] | Level 2 [Member] | U.S. Government Issued Debt [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments	1,855	1,626
Pension Benefits [Member] | Level 2 [Member] | Corporate Bonds Debt [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments	1,151	993
Pension Benefits [Member] | Level 2 [Member] | Structured Assets Debt [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments	4	30
Pension Benefits [Member] | Level 2 [Member] | Other Fixed Income Debt [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments	150	123
Pension Benefits [Member] | Level 2 [Member] | Cash and Cash Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments	558	328
Pension Benefits [Member] | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments	1,335	853
Funds for retiree health benefits	(42)	(31)
Investments (excluding funds for retiree health benefits)	1,293	822
Pension Benefits [Member] | Level 3 [Member] | Private Equity [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments	67	20
Pension Benefits [Member] | Level 3 [Member] | Real Estate [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments	1,062	833
Pension Benefits [Member] | Level 3 [Member] | Hedge Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments	$ 206