Fair Value Measurements and Derivatives - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended	9 Months Ended
	Sep. 30, 2012	Sep. 30, 2012	Dec. 31, 2011	Sep. 30, 2011
Fair Value, Measurement Inputs, Disclosure [Line Items]
Notional amounts of derivatives	$ 233	$ 233	$ 213
Deferred gains (losses) which are reported in AOCI that are expected to be reclassified to earnings during the next twelve months	2	2	(13)
Deferred gains (losses) which are reported in AOCI that are reclassified to earnings during the current period	(1)	(7)
Unobservable Input Risk Premium		2.50%
Excess of callable value over fair value of note				2
Fair Value, Inputs, Level 2 | Market Approach Valuation Technique | Senior Notes
Fair Value, Measurement Inputs, Disclosure [Line Items]
Fair value debt instrument	805	805	765
Fair Value, Inputs, Level 2 | Income Approach Valuation Technique | Other Indebtedness
Fair Value, Measurement Inputs, Disclosure [Line Items]
Fair value debt instrument	$ 116	$ 116	$ 103