Fair Value - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Jun. 30, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Dec. 31, 2011
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	37	36	38	38	40	41
Upper limit percentage of Level 3 assets to total invested assets	0.40%
Long-term debt, carrying value	2,603		2,611
Long-term debt, fair value	$ 2,745		$ 2,923