Fair Value - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2012	Dec. 31, 2011
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Transfers between Level 1 and Level 2	$ 0	$ 0	$ 0	$ 0
Level 3 assets fair value	36	40	36	40	38	38	40	41
Upper limit percentage of Level 3 assets to total invested assets	0.40%		0.40%
Long-term debt, carrying value	2,606		2,606			2,611
Long-term debt, fair value	$ 2,772		$ 2,772			$ 2,923