Income taxes - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2012	Dec. 31, 2013
Income Taxes [Line Items]
Deferred Tax Liabilities, Net	$ 71,773,000		$ 59,357,000
Change in valuation allowance	4,400,000
Federal net operating loss, valuation allowances released		72,600,000
Federal net operating loss, deferred tax benefit		36,600,000
Anticipated refunds received from previously-filed carryback claims	2,800,000
U.S. Federal
Income Taxes [Line Items]
Deferred Tax Liabilities, Net	12,000,000		2,500,000
International
Income Taxes [Line Items]
Deferred Tax Liabilities, Net	700,000		500,000
Federal net operating loss, valuation allowances released			6,600,000
Foreign tax credits carry forwards, deferred tax asset valuation allowances released	5,000,000
Tax credit carry forwards	$ 26,800,000
Tax credit carry forwards, expiration year	2024