Pension Benefits - Fair Value of Plan Assets (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Private Equity [Member]
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	$ 20
Corporate Bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets		94
Structured Assets [Member]
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets		13
Other Fixed Income [Member]
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets		29
Real Estate [Member]
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan net assets	833	572
Pension benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments	9,878	8,516
Funds for retiree health benefits	(353)	(336)
Investments (excluding funds for retiree health benefits)	9,525	8,180
Pending activities	(390)	(380)
Total fair value of plan net assets	9,135	7,800	7,721	6,877
Pension benefits [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments	5,172	4,423
Funds for retiree health benefits	(185)	(174)
Investments (excluding funds for retiree health benefits)	4,987	4,249
Pension benefits [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments	3,853	3,385
Funds for retiree health benefits	(137)	(134)
Investments (excluding funds for retiree health benefits)	3,716	3,251
Pension benefits [Member] | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments	853	708
Funds for retiree health benefits	(31)	(28)
Investments (excluding funds for retiree health benefits)	822	680
Pension benefits [Member] | U.S. Equity [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments	2,637	2,467
Pension benefits [Member] | U.S. Equity [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments	2,637	2,467
Pension benefits [Member] | International Equity [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments	2,995	2,487
Pension benefits [Member] | International Equity [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments	2,242	1,850
Pension benefits [Member] | International Equity [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments	753	637
Pension benefits [Member] | Private Equity [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments	20
Pension benefits [Member] | Private Equity [Member] | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments	20
Pension benefits [Member] | U.S. Government Issues [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments	1,626	1,570
Pension benefits [Member] | U.S. Government Issues [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments	1,626	1,570
Pension benefits [Member] | Corporate Bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments	993	762
Total fair value of plan net assets		94	129
Pension benefits [Member] | Corporate Bonds [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments	993	668
Pension benefits [Member] | Corporate Bonds [Member] | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments		94
Pension benefits [Member] | Structured Assets [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments	30	13
Total fair value of plan net assets		13	87
Pension benefits [Member] | Structured Assets [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments	30
Pension benefits [Member] | Structured Assets [Member] | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments		13
Pension benefits [Member] | Other Fixed Income [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments	123	96
Total fair value of plan net assets		29	66
Pension benefits [Member] | Other Fixed Income [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments	123	67
Pension benefits [Member] | Other Fixed Income [Member] | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments		29
Pension benefits [Member] | Real Estate [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments	833	572
Total fair value of plan net assets		572	398
Pension benefits [Member] | Real Estate [Member] | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments	833	572
Pension benefits [Member] | Cash and Cash Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments	411	408
Pension benefits [Member] | Cash and Cash Equivalents [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments	83	13
Pension benefits [Member] | Cash and Cash Equivalents [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments	328	395
Pension benefits [Member] | Futures [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments	210	141
Pension benefits [Member] | Futures [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments	210	93
Pension benefits [Member] | Futures [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments		$ 48