Income Taxes - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Benefit from income taxes	$ 9,704	$ 11,464	$ 38,909	$ 20,644
Effective income tax benefit rate	27.20%	43.90%	38.40%	46.90%
Increase (decrease) in gross unrecognized tax benefits			$ 5,500
U.S. statutory federal income tax rate, percent			21.00%		35.00%

Percentage of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

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Percentage of domestic federal statutory tax rate applicable to pretax income (loss).

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Amount of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

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Amount of increase (decrease) in unrecognized tax benefits attributable to uncertain tax positions taken in tax returns.

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