Fair Value - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Transfers between Level 1 and Level 2	$ 0	$ 0
Level 3 assets fair value	38	41	66	93
Upper limit percentage of Level 3 assets to total invested assets	0.40%
Long-term debt, carrying value	2,611	1,659
Long-term debt, fair value	$ 2,923	$ 1,834