Income Taxes - Deferred Tax Assets (Liabilities) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred tax assets:
Net operating loss carryforwards	$ 92,315	$ 115,151
Tax credits	12,706	11,180
Stock-based compensation	4,922	7,339
Capitalized research and development	1,126	2,167
Financing liabilities	5,531	5,161
Other	7,613	8,349
Total deferred tax assets	124,213	149,347
Deferred tax liabilities:
Fixed assets	(5,485)	(5,754)
Intangible assets	(1,902)	(3,606)
Other	(222)	(187)
Total deferred tax liabilities	(7,609)	(9,547)
Net deferred tax assets before valuation allowance	116,604	139,800
Less valuation allowance	(115,035)	(138,795)
Net deferred tax assets	$ 1,569	$ 1,005