Derivative Instruments and Hedging Activities - Impact of Periodic Settlements of Active Swap Agreements and Impact of Terminated Swap Agreements on Results of Operations (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Interest Rate Swap Agreements [Line Items]
Periodic settlements of active swap agreements	$ 8	$ 23	$ 29
Terminated swap agreements	22	12	18
Total	$ 30	$ 35	$ 47