Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Current assets:
Cash and cash equivalents	$ 56,356	$ 52,069
Investments, available-for-sale	150,040	162,090
Unbilled accounts receivable	2,826	3,577
Prepaid expenses and other current assets	4,736	2,689
Total current assets	213,958	220,425
Investments, available-for-sale	29,929	54,059
Property and equipment, net	6,081	6,188
Other assets	1,413	856
Restricted cash	1,267	1,267
Total assets	252,648	282,795
Current liabilities:
Accounts payable	2,282	2,211
Accrued expenses	10,346	11,746
Current portion of deferred revenue	10,396	11,426
Current portion of lease incentive obligation	578	578
Current portion of deferred rent	17
Current portion of term loan payable	2,140	2,551
Total current liabilities	25,759	28,512
Deferred rent, net of current portion	924	932
Deferred revenue, net of current portion	33,824	35,809
Lease incentive obligation, net of current portion	2,648	2,792
Term loan payable, net of current portion	1,106	1,518
Other long term liabilities	165	154
Commitments (Note 10)
Stockholders' deficit:
Preferred stock, $0.001 par value; 5,000,000 shares authorized; no shares issued and outstanding
Common stock, $0.001 par value; 120,000,000 shares authorized; 33,249,614 and 33,125,479 shares issued at March 31, 2017 and December 31, 2016, respectively, and 33,249,360 and 33,123,354 shares outstanding at March 31, 2017 and December 31, 2016, respectively	33	33
Additional paid-in capital	423,750	420,533
Accumulated other comprehensive loss	(114)	(18)
Accumulated deficit	(235,447)	(207,470)
Total stockholders' equity	188,222	213,078
Total liabilities and stockholders' equity	$ 252,648	$ 282,795