Income taxes - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended	6 Months Ended
Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Dec. 31, 2013
Income Tax Disclosure [Abstract]
Income tax expense at federal statutory rate, rate	35.00%	35.00%
Unrecognized income tax benefits	$ 21.4		$ 21.4		$ 21.6
Recognized additional liabilities			0.90	0.47
Reversed prior year liabilities			$ 1.1

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

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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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Amount of decrease in unrecognized tax benefits resulting from tax positions taken in prior period tax returns.

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Amount of increase in unrecognized tax benefits resulting from tax positions that have been or will be taken in current period tax return.

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