Income taxes - Reconciliation of Income Tax Expenses Computed at Federal Statutory Rate with Income Tax Expense (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Tax Disclosure [Abstract]
Expected income tax expense (benefit) at 35%	$ 26,279	$ (194)	$ 15,140
Return-to-provision and true-up adjustments	(4,127)	(1,329)	1,133
Previously unrecognized research and development credits	(3,434)	0	0
Nondeductible expenses	2,772	2,768	4,060
Intercompany dividends, net of deduction for dividends received	(539)	(541)	(319)
Tax-exempt interest	(458)	(404)	(324)
State income tax benefit – net of Federal impact	(407)	(914)	(840)
Noncontrolling interest	303	251	186
Valuation allowance	294	(668)	(5,020)
Foreign tax rate differential	(207)	(329)	(693)
Impairment of goodwill	0	7,099	0
Other – net	(871)	(89)	180
Income tax expense	$ 19,605	$ 5,650	$ 13,503
Effective income tax rates	26.10%	(1019.40%)	31.20%