Income Taxes - Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Operating Loss Carryforwards [Line Items]
Income tax provision (benefit)	$ (3)	$ 314	$ 458	$ 756
Net loss before income taxes	$ 82,513	$ 76,044	$ 154,588	$ 208,588
Effective income tax rate	0.00%	(0.40%)	(0.30%)	(0.40%)
Valuation allowance					$ 542,400
Statutory tax rate	21.00%	34.00%
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