Income taxes - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Components Of Income Tax Expense Benefit [Line Items]
Valuation allowance	$ 26,125,000	$ 19,115,000
Net increase in valuation allowance	7,000,000
Operating loss carry forwards, state, net of tax	21,500,000
Operating loss carry forwards, foreign, net of tax	101,600,000
Income tax expense additional deduction related to the U.S Government resolutions	1,300,000
Income tax expense	28,792,000	21,181,000	22,606,000
Income tax effective rate	34.80%	108.90%	44.90%
Income tax effective rate excluding the impact of the charges related to the U.S Government resolutions (as a percent)	36.00%	38.00%
Gross unrecognized tax benefit	1,000,000	600,000
Accrued interest and penalties related to unrecognized tax benefits	600,000	500,000
Unremitted earnings of foreign subsidiaries on which income tax expense is not provided	$ 248,000,000