Income Taxes Income Taxes - Schedule of Deferred Income Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred tax assets:
Net operating loss carryforwards	$ 40,502	$ 57,327
Stock—based compensation	2,097	2,863
Development costs	27,213	26,805
Other	1,396	1,939
Net deferred tax assets	71,208	88,934
Valuation allowance	(71,208)	(88,934)
Deferred tax assets, net	$ 0	$ 0