Income Taxes - Effective Income Tax Rate Reconciliation (Details)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Effective income tax rate reconciliation
U.S. Federal statutory tax rate	(35.00%)	(35.00%)	(35.00%)
Deferred state taxes, net of federal benefit	(3.10%)	(1.20%)	(1.30%)
Common stock warrant liability	(2.30%)	20.60%	20.60%
Other, net	0.30%	(0.20%)	0.60%
Change in valuation allowance	40.10%	15.40%	14.50%
Total effective income tax rate	0.00%	(0.40%)	(0.60%)