Income Taxes - Schedule of Temporary Differences That Give Rise to Significant Net Deferred Tax Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Deferred tax assets
Net operating losses	$ 43,133	$ 194,027
Capitalized research and development expenses	847	15,854
Credit carryforwards	164,709	144,823
Depreciation	2,605	2,557
Deferred compensation	5,855	12,463
Accrued expenses	905	3,601
Deferred revenue		21,935
Other temporary differences	2,145	25,276
Installment sale tax basis	15,868
Total gross deferred tax assets	236,067	420,536
Valuation allowance	$ (236,067)	(414,558)	$ (326,577)	$ (257,489)
Net deferred tax assets		5,978
Deferred tax liabilities
Intangible assets		(1,428)
Debt discount		$ (4,550)