Income Taxes - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Tax Credit Carryforward [Line Items]
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Amount		$ 0	$ 70,055	$ 0
Significant Change in Unrecognized Tax Benefits is Reasonably Possible, Amount of Unrecorded Benefit		$ 700
Statutory federal income tax rate		21.00%	35.00%	35.00%
Goodwill, Written off Related to Asset Divestiture	$ 14,008		$ 0
Change for uncertain tax positions		$ (600)
Deferred tax assets, gross		56,251	53,530
Deferred Tax Assets, Valuation Allowance		3,764	3,733
Proceeds and Excess Tax Benefit from Share-based Compensation		700	2,200
Change in valuation allowance			700
Unrecognized Tax Benefits, Income Tax Penalties and Interest Accrued		2,800
Unrecognized Tax Benefits		2,785	2,770
Unrecognized tax benefits , if recognized, would favorably impact effective tax rate		2,800	3,600	$ 4,100
Accrued interest related to tax obligations		100	100	100
Federal and state credit carryforwards		10,705	11,752
Investment Tax Credit		10,000	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		$ 8,900