Income taxes - Additional Information (Detail) (USD $)	3 Months Ended	9 Months Ended
Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011
Income Tax [Line Items]
Provision for income tax	$ 6,930,000	$ 7,982,000	$ 20,286,000	$ 20,038,000
Income tax effective rate (as a percent)			34.00%	296.10%
Income tax effective rate excluding the impact of the charges related to the U.S. Government resolutions (as a percent)			36.60%	38.00%
Gross unrecognized tax benefit	600,000		600,000		700,000
Accrued interest and penalties related to unrecognized tax benefits	$ 500,000		$ 500,000		$ 500,000

A ratio calculated by dividing the reported amount of income tax expense attributable to continuing operations for the period by GAAP-basis pretax income from continuing operations.

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The sum of the current income tax expense or benefit and the deferred income tax expense or benefit pertaining to continuing operations.

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- Definition

This element represents the total of accruals as of the date of the statement of financial position for interest recognized for an underpayment of income taxes computed by applying the applicable statutory rate of interest to the difference between a tax position recognized for financial reporting purposes and the amount previously taken or expected to be taken in a tax return of the entity and the amount of statutory penalties for a tax position claimed or expected to be claimed by the entity, in its tax return, that does not meet the minimum statutory threshold to avoid payment of penalties.

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