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Income Taxes (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Current tax expense:
State income tax	$ 1	$ 1	$ 2
Canadian federal and provincial income tax	70us-gaap_CurrentForeignTaxExpenseBenefit	99us-gaap_CurrentForeignTaxExpenseBenefit	51us-gaap_CurrentForeignTaxExpenseBenefit
Total current tax expense	71us-gaap_CurrentIncomeTaxExpenseBenefit	100us-gaap_CurrentIncomeTaxExpenseBenefit	53us-gaap_CurrentIncomeTaxExpenseBenefit
Deferred tax (benefit)/expense:
Canadian federal and provincial income tax	100	(1)	1
Total deferred tax (benefit)/expense	100us-gaap_DeferredIncomeTaxExpenseBenefit	(1)us-gaap_DeferredIncomeTaxExpenseBenefit	1us-gaap_DeferredIncomeTaxExpenseBenefit
Total income tax expense	$ 171us-gaap_IncomeTaxExpenseBenefit	$ 99us-gaap_IncomeTaxExpenseBenefit	$ 54us-gaap_IncomeTaxExpenseBenefit

Amount of current income tax expense (benefit) pertaining to taxable income (loss) from continuing operations.

Amount of current state and local tax expense (benefit) pertaining to income (loss) from continuing operations.

Amount of deferred foreign income tax expense (benefit) pertaining to income (loss) from continuing operations.

Amount of deferred income tax expense (benefit) pertaining to income (loss) from continuing operations.