Income taxes - Reconciliation of Income Tax Expenses Computed at Federal Statutory Rate with Income Tax Expense (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Tax Disclosure [Abstract]
Expected income tax expense at 35%	$ 15,140	$ 32,027	$ 27,842
Foreign tax rate differential	(693)	(422)	(2,688)
State income tax (benefit) expense – net of Federal impact	(840)	1,586	1,147
Non-deductible expenses	4,060	2,321	2,851
Non-deductible bond conversion costs	0	1,900	0
Loss carrybacks	0	0	(1,442)
Adjustments to deferred tax liabilities	1,016	(2,408)	12,381
Valuation allowance	(5,020)	(6,555)	(71,106)
Other – net	(160)	32	1,376
Income tax expense (benefit)	$ 13,503	$ 28,481	$ (29,639)
Effective income tax rates (%)	31.20%	31.10%	(37.30%)