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	3 Months Ended				9 Months Ended
	Sep. 30, 2012		Sep. 30, 2011		Sep. 30, 2012		Sep. 30, 2011
Outstanding Interest Rate Swap Agreements [Line Items]
Periodic settlements of active swap agreements			$ 7	[1],[2]	$ 7	[1],[2]	$ 18	[1],[2]
Terminated swap agreements	7	[1]	2	[1]	16	[1]	8	[1]
Total	$ 7		$ 9		$ 23		$ 26

[1] Due to our election to terminate our interest rate swap portfolio with a notional amount of $1 billion in April 2012, periodic settlements of active swap agreements have decreased and amortization to interest expense of terminated swap agreements has increased.

[2] These amounts represent the net of our periodic variable-rate interest obligations and the swap counterparties' fixed-rate interest obligations. Our variable-rate obligations were based on a spread from the three-month LIBOR.