Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 117,751	$ 400,304
Investments, available-for-sale	495,705	273,052
Prepaid expenses and other current assets	8,757	12,149
Total current assets	622,213	685,505
Investments, available-for-sale	3,195
Property and equipment, net	30,250	24,363
Restricted cash	4,557	4,555
Other assets	4,898	1,314
Total assets	665,113	715,737
Current liabilities:
Accounts payable	2,980	3,744
Accrued expenses	39,808	30,541
Current portion of deferred revenue	5,139	5,373
Current portion of lease incentive obligation	1,714	1,714
Current portion of term loan payable	692	1,518
Total current liabilities	50,333	42,890
Deferred rent, net of current portion	4,648	4,129
Deferred revenue, net of current portion	43,156	30,000
Lease incentive obligation, net of current portion	13,760	14,617
Other long term liabilities	284	131
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; 43,883,774 and 43,577,526 shares issued and outstanding at June 30, 2018 and December 31, 2017, respectively;	44	43
Additional paid-in capital	997,770	979,785
Accumulated other comprehensive loss	(381)	(269)
Accumulated deficit	(444,501)	(355,589)
Total stockholders’ equity	552,932	623,970
Total liabilities and stockholders’ equity	$ 665,113	$ 715,737