Income Taxes - Additional Information (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Tax Credit Carryforward [Line Items]
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Amount	$ 70,055	$ 0	$ 0
Federal statutory tax rate under Tax Cuts and Jobs Act	0.21
Significant Change in Unrecognized Tax Benefits is Reasonably Possible, Amount of Unrecorded Benefit	$ 900
Statutory federal income tax rate	35.00%
Change for uncertain tax positions	$ (1,300)
Deferred tax assets, gross	53,530	75,783
Deferred Tax Assets, Valuation Allowance	3,733	4,407
Proceeds and Excess Tax Benefit from Share-based Compensation	2,200
Change in valuation allowance	700	600
Unrecognized Tax Benefits, Income Tax Penalties and Interest Accrued	2,800
Unrecognized Tax Benefits	2,770	2,434
Unrecognized tax benefits , if recognized, would favorably impact effective tax rate	3,600	4,100	3,200
Accrued interest related to tax obligations	100	100	100
Federal and state credit carryforwards	11,752	8,369
Investment Tax Credit	2,400
Income Tax Examination, Penalties Accrued	0	$ 0	$ 0
Valuation allowances, State Tax Credits [Member]
Tax Credit Carryforward [Line Items]
Change in valuation allowance	$ 11,100