Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Current assets:
Cash and cash equivalents	$ 288,678	$ 113,938
Investments, available-for-sale	233,120	369,616
Accounts receivable, net	3,403	663
Unbilled accounts receivable	20,877	22,749
Inventory	4,359
Prepaid expenses and other current assets	17,875	9,820
Total current assets	568,312	516,786
Investments, available-for-sale	128,475	64,406
Property and equipment, net	36,745	38,361
Operating lease right-of-use assets, net	70,471	72,753
Restricted cash	5,168	5,166
Other assets	8,286	10,222
Total assets	817,457	707,694
Current liabilities:
Accounts payable	5,591	4,793
Accrued expenses	84,062	88,706
Current portion of operating lease liabilities	7,416	6,823
Current portion of deferred revenue	8,017	6,160
Total current liabilities	105,086	106,482
Operating lease liabilities, net of current portion	85,727	89,126
Deferred revenue, net of current portion	36,688	39,913
Other long-term liabilities	8,589	7,814
Total liabilities	236,090	243,335
Commitments (Note 15)
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; 54,284,501 and 49,272,223 shares issued and outstanding at June 30, 2020 and December 31, 2019, respectively	54	49
Additional paid-in capital	1,762,099	1,412,083
Accumulated other comprehensive loss	(1,120)	(2,535)
Accumulated deficit	(1,179,666)	(945,238)
Total stockholders' equity	581,367	464,359
Total liabilities and stockholders' equity	$ 817,457	$ 707,694