Income Taxes - Additional Information (Detail) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2017 USD ($)	Sep. 30, 2017 USD ($)
Tax Credit Carryforward [Line Items]
Deferred tax assets, net	$ 22.8	$ 22.8
Research and development tax credit studies		17.1
Unrecognized tax benefits	4.3	$ 4.3
Overall effective tax rate, after discrete period adjustments		16.30%
Effective tax rate, before discrete period adjustment		32.50%
Discrete tax benefit, stock-based compensation		$ 1.5
Unrecognized tax benefits, reduction resulting from lapse of statute of limitations	0.7
Effective income tax reconciliation, discrete tax benefit, net tax return true-ups, amount	0.4
Federal
Tax Credit Carryforward [Line Items]
Unrecognized tax benefits	4.2	4.2
State
Tax Credit Carryforward [Line Items]
Unrecognized tax benefits	0.1	0.1
Expiring in 2019
Tax Credit Carryforward [Line Items]
Tax credit carryforward, amount	1.2	1.2
Research and Development Credits
Tax Credit Carryforward [Line Items]
Tax credit carryforward, valuation allowance	$ 1.8	$ 1.8