CONDENSED CONSOLIDATED BALANCE SHEETS (Unaudited) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
CURRENT ASSETS:
Cash and cash equivalents	$ 245,479	$ 152,816
Held-to-maturity investments	461,411	196,065
Contract receivables	15,000	200,354
Prepaid expenses and other current assets	8,606	6,934
Total current assets	730,496	556,169
NONCURRENT ASSETS:
Property and equipment, net	7,500	7,076
Right-of-use operating assets, net	29,932	30,102
Restricted cash equivalents	5,563	3,894
Other noncurrent assets	5,298	168
Total noncurrent assets	48,293	41,240
TOTAL ASSETS	778,789	597,409
CURRENT LIABILITIES:
Accounts payable	6,497	6,077
Accrued expenses and other current liabilities	19,087	20,042
Lease liability, current	3,164	3,358
Deferred revenue, current	223,556	212,258
Total current liabilities	252,304	241,735
NONCURRENT LIABILITIES:
Lease liability, noncurrent	20,518	20,141
Deferred revenue, noncurrent	327,506	182,730
Other noncurrent liabilities	555	608
Total noncurrent liabilities	348,579	203,479
TOTAL LIABILITIES	600,883	445,214
COMMITMENTS AND CONTINGENCIES (NOTE 8)
STOCKHOLDERS’ EQUITY:
Preferred stock, $0.0001 par value – 5,000,000 shares authorized; no shares issued or outstanding at March 31, 2020 or December 31, 2019	0	0
Common stock, $0.0001 par value – 150,000,000 shares authorized; 73,779,069 and 71,573,196 shares issued and outstanding at March 31, 2020 and December 31, 2019, respectively	7	7
Additional paid-in capital	725,707	677,504
Accumulated deficit	(547,808)	(525,316)
Total stockholders’ equity	177,906	152,195
TOTAL LIABILITIES AND STOCKHOLDERS’ EQUITY	$ 778,789	$ 597,409