Income Taxes - Rate reconciliation (Details)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Federal tax benefit at statutory rate	34.00%	34.00%	34.00%
State tax benefit net of federal effect	2.20%	2.80%	1.10%
Foreign taxes	(0.40%)	(0.30%)	(0.60%)
Change in valuation allowance	(33.90%)	(35.00%)	(36.30%)
Credits	1.50%	1.90%	6.10%
Stock-based compensation	(4.10%)	(3.50%)	(4.50%)
Non-deductible expenses and other	(0.40%)	(0.70%)	(0.70%)
Effective tax rate	(1.10%)	(0.80%)	(0.90%)