Income Taxes - Rate reconciliation (Details)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
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	4.40%	2.20%	2.80%
Foreign taxes	(0.40%)	(0.40%)	(0.30%)
Change in valuation allowance	(33.00%)	(33.90%)	(35.00%)
Credits	1.80%	1.50%	1.90%
Stock-based compensation	(8.00%)	(4.10%)	(3.50%)
Non-deductible expenses and other	(0.30%)	(0.40%)	(0.70%)
Effective tax rate	(1.50%)	(1.10%)	(0.80%)