Income Taxes - Deferred (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Deferred tax assets:
Net operating loss carryforwards		$ 118,672	$ 167,869
Research and development tax credit carryforwards		58,606	43,096
Property and other intangible assets		2,272	2,982
Deferred revenue		25,997	13,205
Stock-based compensation		12,125	16,794
Deferred lease incentive		2,889	4,264
Other liabilities		3,037	2,107
Royalty sale		47,143	73,973
Total deferred tax assets		270,741	324,290
Deferred tax liabilities:
Royalty sale transaction costs		(859)	(1,569)
Total deferred tax liabilities		859	1,569
Valuation allowance		(269,882)	$ (322,721)
Income taxes, additional disclosures
Increase (decrease) in valuation allowance		$ (52,800)
U.S. federal corporate tax rate (as a percent)		34.00%
Provisional income tax expense, Tax Cuts and Jobs Act of 2017		$ 97,500
Forecast
Income taxes, additional disclosures
U.S. federal corporate tax rate (as a percent)	21.00%
Maximum
Income taxes, additional disclosures
U.S. federal corporate tax rate (as a percent)		35.00%