Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Current assets:
Cash and cash equivalents	$ 57,329	$ 52,069
Investments, available-for-sale	333,323	162,090
Unbilled accounts receivable	3,435	3,577
Prepaid expenses and other current assets	6,192	2,689
Total current assets	400,279	220,425
Investments, available-for-sale		54,059
Property and equipment, net	13,416	6,188
Other assets	1,656	856
Restricted cash	4,767	1,267
Total assets	420,118	282,795
Current liabilities:
Accounts payable	2,092	2,211
Accrued expenses	23,797	11,746
Current portion of deferred revenue	6,069	11,426
Current portion of lease incentive obligation	1,714	578
Current portion of term loan payable	1,818	2,551
Total current liabilities	35,490	28,512
Deferred rent, net of current portion	3,866	932
Deferred revenue, net of current portion	30,703	35,809
Lease incentive obligation, net of current portion	8,186	2,792
Term loan payable, net of current portion	277	1,518
Other long term liabilities	356	154
Commitments (Note 10)
Stockholders’ equity:
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; 39,179,019 and 33,125,479 shares issued at September 30, 2017 and December 31, 2016, respectively, and 39,179,019 and 33,123,354 shares outstanding at September 30, 2017 and December 31, 2016, respectively	39	33
Additional paid-in capital	647,928	420,533
Accumulated other comprehensive loss	(165)	(18)
Accumulated deficit	(306,562)	(207,470)
Total stockholders' equity	341,240	213,078
Total liabilities and stockholders' equity	$ 420,118	$ 282,795