Income Taxes - Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Operating Loss Carryforwards [Line Items]
Income tax provision	$ 128	$ 228	$ 461	$ 442
Net loss before income taxes	$ 50,060	$ 67,370	$ 72,075	$ 132,544
Effective income tax rate	(0.30%)	(0.30%)	(0.60%)	(0.30%)
Valuation allowance					$ 542,400
Domestic Tax Authority
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards					1,700,000
State and Local Jurisdiction
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards					1,500,000
Research Tax Credit Carryforward | Domestic Tax Authority
Operating Loss Carryforwards [Line Items]
Tax credit carryforwards					16,100
Research Tax Credit Carryforward | State and Local Jurisdiction
Operating Loss Carryforwards [Line Items]
Tax credit carryforwards					12,200
Investment Tax Credit Carryforward | Domestic Tax Authority
Operating Loss Carryforwards [Line Items]
Tax credit carryforwards					$ 6,600