CONDENSED CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
CURRENT ASSETS:
Cash and cash equivalents	$ 204,281	$ 152,816
Held-to-maturity investments	405,632	196,065
Contract receivables	5,028	200,354
Prepaid expenses and other current assets	12,710	6,934
Total current assets	627,651	556,169
NONCURRENT ASSETS:
Property and equipment, net	12,846	7,076
Right-of-use operating assets, net	28,136	30,102
Restricted cash equivalents	6,362	3,894
Other noncurrent assets	6,743	168
Total noncurrent assets	54,087	41,240
TOTAL ASSETS	681,738	597,409
CURRENT LIABILITIES:
Accounts payable	7,249	6,077
Accrued expenses and other current liabilities	30,050	20,042
Lease liability, current	2,428	3,358
Deferred revenue, current	200,041	212,258
Total current liabilities	239,768	241,735
NONCURRENT LIABILITIES:
Lease liability, noncurrent	19,160	20,141
Deferred revenue, noncurrent	268,502	182,730
Other noncurrent liabilities	608	608
Total noncurrent liabilities	288,270	203,479
TOTAL LIABILITIES	528,038	445,214
COMMITMENTS AND CONTINGENCIES (NOTE 11)
STOCKHOLDERS’ EQUITY:
Preferred stock, $0.0001 par value – 5,000,000 shares authorized; no shares issued or outstanding at September 30, 2020 or December 31, 2019	0	0
Common stock, $0.0001 par value – 150,000,000 shares authorized; 74,680,999 and 71,573,196 shares issued and outstanding at September 30, 2020 and December 31, 2019, respectively	7	7
Additional paid-in capital	755,170	677,504
Accumulated deficit	(601,477)	(525,316)
Total stockholders’ equity	153,700	152,195
TOTAL LIABILITIES AND STOCKHOLDERS’ EQUITY	$ 681,738	$ 597,409