Income Taxes - Schedule of Tax Effect of Temporary Differences to Significant Portions of Deferred Tax Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2012	Dec. 31, 2011
Deferred tax assets:
Loss carryforwards	$ 154,949	$ 102,960	$ 200	$ 200
Tax credits	119,542	78,324
Stock options	21,336	16,084
Accruals and reserves	5,939	10,039
Fixed assets and intangibles	1,194	3,014
Other	1,286	1,950
Gross deferred tax assets	304,246	212,371
Valuation allowance	(304,246)	$ (212,371)
Deferred tax liabilities:
Net deferred tax assets (liabilities)	(31,166)
In-process Research and Development
Deferred tax liabilities:
In-process research and development	$ (31,166)