Income Taxes - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Tax Disclosure [Line Items]
U.S. federal statutory income tax rate	35.00%	35.00%	35.00%
Benefit over effective income tax rate	$ 15.2	$ 15.2	$ 15.2
Unrecognized tax benefits	16.8us-gaap_UnrecognizedTaxBenefits	16.8us-gaap_UnrecognizedTaxBenefits	16.8us-gaap_UnrecognizedTaxBenefits
Estimated unrecognized tax benefits (decrease)	4.6
Canadian Income Tax Audit
Income Tax Disclosure [Line Items]
Income tax expense (benefit) adjustment	3.0
U.S. Income Tax Audit
Income Tax Disclosure [Line Items]
Income tax expense (benefit) adjustment	$ 3.1

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Percentage of domestic federal statutory tax rate applicable to pretax income (loss).

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Amount of (increase) decrease reasonably possible in the estimated upper bound of the unrecognized tax benefit for a tax position taken or expected to be taken.

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Amount of increase (decrease) to previously recorded tax expense. Includes, but is not limited to, significant settlements of income tax disputes, and unusual tax positions or infrequent actions taken by the entity, including tax assessment reversal, and IRS tax settlement.

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Amount of unrecognized tax benefits pertaining to uncertain tax positions taken in tax returns.

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-Glossary Unrecognized Tax Benefit

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The total amount of unrecognized tax benefits that, if recognized, would affect the effective tax rate.

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