Income Taxes - Additional Information (Details) - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating Loss Carryforwards [Line Items]
Income tax provision	$ 1,079,000	$ (3,000)	$ 128,000	$ 333,000	$ (120,000)	$ 314,000	$ 228,000	$ 214,000	$ 1,537,000	$ 636,000	$ 729,000
Net loss before income taxes	103,364,000	$ 82,513,000	$ 50,060,000	$ 22,015,000	72,041,000	$ 76,044,000	$ 67,370,000	$ 65,174,000	$ 257,952,000	$ 280,629,000	$ 335,587,000
Effective income tax rate									(0.60%)	(0.20%)	(0.20%)
Valuation allowance	566,442,000				542,409,000				$ 566,442,000	$ 542,409,000
Increase (decrease) in valuation allowance									(24,000,000)	141,300,000
Unrecognized tax benefits that would result in adjustments to the valuation allowance	27,700,000								27,700,000
Interest and penalties accrued	0				$ 0				0	$ 0
Uncertain tax positions increase									1,900,000
Change to provisional amount									600,000
Domestic Tax Authority
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards	1,700,000,000								1,700,000,000
State and Local Jurisdiction
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards	1,500,000,000								1,500,000,000
Research Tax Credit Carryforward | Domestic Tax Authority
Operating Loss Carryforwards [Line Items]
Tax credit carryforwards	17,700,000								17,700,000
Research Tax Credit Carryforward | State and Local Jurisdiction
Operating Loss Carryforwards [Line Items]
Tax credit carryforwards	12,600,000								12,600,000
Investment Tax Credit Carryforward | Domestic Tax Authority
Operating Loss Carryforwards [Line Items]
Tax credit carryforwards	$ 6,600,000								$ 6,600,000