Pension Benefits - Reconciliation of Net Balances (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended												12 Months Ended
	Dec. 31, 2012 Real Estate [Member]	Dec. 31, 2012 Corporate Bonds [Member]	Dec. 31, 2012 Other Fixed Income [Member]	Dec. 31, 2012 Structured Assets [Member]	Dec. 31, 2012 Investments [Member]	Dec. 31, 2012 Funds for retiree health benefits [Member]	Dec. 31, 2012 Investments (excluding funds for retiree health benefits) [Member]	Dec. 31, 2012 Private Equity [Member]	Dec. 31, 2012 Pension benefits [Member]	Dec. 31, 2011 Pension benefits [Member]	Dec. 31, 2010 Pension benefits [Member]	Dec. 31, 2009 Pension benefits [Member]	Dec. 31, 2011 Pension benefits [Member] Real Estate [Member]	Dec. 31, 2011 Pension benefits [Member] Corporate Bonds [Member]	Dec. 31, 2011 Pension benefits [Member] Other Fixed Income [Member]	Dec. 31, 2011 Pension benefits [Member] Structured Assets [Member]	Dec. 31, 2011 Pension benefits [Member] Investments [Member]	Dec. 31, 2011 Pension benefits [Member] Funds for retiree health benefits [Member]	Dec. 31, 2011 Pension benefits [Member] Investments (excluding funds for retiree health benefits) [Member]
Defined Benefit Plan Disclosure [Line Items]
Beginning Balance as of January 1, 2011	$ 572	$ 94	$ 29	$ 13	$ 708	$ (28)	$ 680		$ 9,135	$ 7,800	$ 7,721	$ 6,877	$ 398	$ 129	$ 66	$ 87	$ 680	$ (30)	$ 650
Assets Still Held at Reporting Date - Unrealized Gains/(Losses)	48				49	(2)	47	1					65	(9)	(1)	(1)	54	3	57
Assets Sold During the Period - Realized Gains	1			(6)	(5)		(5)							11	3	2	16	1	17
Purchases Sales and Settlements	212	(33)	(6)		192	(4)	188	19					109	(37)	(39)	(75)	(42)	(2)	(44)
Transfer in/(out) of Level 3		(61)	(23)	(7)	(91)	3	(88)
FAIR VALUE OF PLAN ASSETS AT END OF YEAR	$ 833				$ 853	$ (31)	$ 822	$ 20	$ 9,135	$ 7,800	$ 7,721	$ 6,877	$ 572	$ 94	$ 29	$ 13	$ 708	$ (28)	$ 680