Other Postretirement Benefits - Reconciliation of Fair Value Balances for Net Assets (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended						12 Months Ended
	Dec. 31, 2012 Other Fixed Income [Member]	Dec. 31, 2012 Funds for retiree health benefits [Member]	Dec. 31, 2012 Other Postretirement Benefits [Member]	Dec. 31, 2011 Other Postretirement Benefits [Member]	Dec. 31, 2010 Other Postretirement Benefits [Member]	Dec. 31, 2009 Other Postretirement Benefits [Member]	Dec. 31, 2012 Other Postretirement Benefits [Member] Level 3 [Member]	Dec. 31, 2011 Other Postretirement Benefits [Member] Level 3 [Member]	Dec. 31, 2012 Other Postretirement Benefits [Member] Level 3 [Member] Other Fixed Income [Member]	Dec. 31, 2011 Other Postretirement Benefits [Member] Level 3 [Member] Other Fixed Income [Member]	Dec. 31, 2012 Other Postretirement Benefits [Member] Level 3 [Member] Insurance Contracts [Member]	Dec. 31, 2012 Other Postretirement Benefits [Member] Level 3 [Member] Funds for retiree health benefits [Member]	Dec. 31, 2011 Other Postretirement Benefits [Member] Level 3 [Member] Funds for retiree health benefits [Member]	Dec. 31, 2012 Other Postretirement Benefits [Member] Level 3 [Member] Investments (including funds for retiree health benefits) [Member]	Dec. 31, 2011 Other Postretirement Benefits [Member] Level 3 [Member] Investments (including funds for retiree health benefits) [Member]
Schedule Of Other Postretirement Benefits [Line Items]
Beginning Balance as of January 1, 2011	$ 29	$ (28)	$ 1,047	$ 947	$ 942	$ 866	$ 28	$ 219		$ 189			$ 189	$ 28	$ 30
Assets Still Held at Reporting Date - Unrealized Gains/(Losses)		(2)					2	(3)						2	(3)
Assets Sold During the Period - Realized(Losses)								(1)							(1)
Purchases Sales and Settlements	(6)	(4)					4	2						4	2
Transfers Out of Level 3							(3)	(189)		(189)			(189)	(3)
Ending Balance as of December 31, 2011		$ (31)	$ 1,047	$ 947	$ 942	$ 866	$ 31	$ 28						$ 31	$ 28