Accumulated Other Comprehensive Income - Other Comprehensive Income (Loss) Before Reclassifications Associated With Cash Flow Derivative Instruments (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss) on derivatives before tax	$ 10	$ 23	$ (36)
Tax (expense) benefit	(4)	(9)	14
Net of tax	6	14	(22)
Interest Rate Swaps [Member] | Cash Flow Hedging [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss) on derivatives before tax	(8)	14	(27)
Foreign Currency Derivatives [Member] | Cash Flow Hedging [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss) on derivatives before tax	23	17	(9)
Electricity Commodity Derivatives [Member] | Cash Flow Hedging [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss) on derivatives before tax	$ (5)	$ (8)