Income Taxes - Variation Between Expected and Effective Tax Rate (Detail)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Statutory federal income tax (benefit) rate	35.00%	35.00%	35.00%
State income taxes (net of federal benefit)	2.50%	5.70%	1.20%
Qualified domestic production activities	(2.60%)	(2.00%)	(0.50%)
Stock-based compensation expense	(8.20%)	0.00%	0.00%
Research and development tax credits	(50.60%)	(8.60%)	2.90%
Other tax credits	(7.50%)	(0.00%)	(0.00%)
Goodwill impairment	0.00%	0.00%	(8.10%)
Changes in valuation allowance	10.60%	0.90%	(0.60%)
Non-deductible book expenses	1.10%	0.20%	(0.20%)
Changes in deferred tax assets	15.40%	1.50%	(0.10%)
Re-measurement of deferred taxes for 2017 Tax Act	(171.30%)	0.00%	0.00%
Changes in tax reserves	11.40%	0.00%	(0.10%)
Other	0.40%	1.00%	0.20%
Effective income tax (benefit) rate	(163.80%)	33.70%	29.70%