Income Taxes - Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Taxes
Income tax expense (benefit)	$ 0	$ 0	$ 0
Unrecognized tax benefit, which, if recognized, will not affect effective tax rate	1,300,000
Penalties and interest expense	0	0	$ 0
TCJA decrease in deferred tax assets	82,800,000
Federal
Income Taxes
Net operating loss carryforwards	751,700,000
Tax credits	4,400,000
Operating loss carryforwards from excess stock based compensation		16,100,000
State
Income Taxes
Net operating loss carryforwards	669,300,000
Tax credits	1,200,000
Research Tax Credit Carryforward
Income Taxes
Tax credits	$ 200,000
Accounting Standards Update 2016-09
Income Taxes
Deferred tax assets		6,100,000
Deferred tax assets, valuation allowance		$ 6,100,000