Income Taxes - Schedule of Temporary Differences That Give Rise to Significant Net Deferred Tax Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Deferred tax assets
Net operating losses	$ 54,407	$ 43,133
Capitalized research and development expenses	705	847
Credit carryforwards	166,367	164,709
Depreciation	2,725	2,605
Deferred compensation	6,315	5,855
Accrued expenses	655	905
Other temporary differences	2,423	2,145
Installment sale tax basis	14,881	15,868
Total gross deferred tax assets	248,478	236,067
Valuation allowance	$ (248,478)	$ (236,067)	$ (414,558)