Employee Benefits - Changes in Plan Assets and Benefit Obligations Recognized in Other Comprehensive Income (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Pension Benefits [Member]
Changes in plan assets and benefit obligations recognized in other comprehensive income
Net actuarial loss (gain)	$ 139	$ 355	$ 16
Amortization of net actuarial loss and prior service cost and other	(80)	(68)	(64)
Total Changes in plan assets and benefit obligations recognized in other comprehensive income	59	287	(48)
Other Postretirement Benefits [Member]
Changes in plan assets and benefit obligations recognized in other comprehensive income
Net actuarial loss (gain)	(11)	45	30
Amortization of net actuarial loss and prior service cost and other	(3)	(3)	(2)
Total Changes in plan assets and benefit obligations recognized in other comprehensive income	$ (14)	$ 42	$ 28