CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
CASH FLOWS FROM OPERATING ACTIVITIES:
Net loss	$ (22,492)	$ (26,154)
Adjustments to reconcile net loss to net cash provided by operating activities:
Stock-based compensation expense	8,527	4,827
Depreciation and amortization expense	535	242
Amortization of premium on investments	(191)	(1,225)
Non-cash lease expense	1,803	445
Changes in operating assets and liabilities:
Litigation settlement payable	0	(10,500)
Deferred revenue	156,074	4,893
Prepaid expenses and other assets	(6,802)	886
Accounts payable	957	1,064
Contract receivables	185,354	100,000
Accrued expenses and other liabilities	(488)	(1,735)
Lease liability	(2,131)	(456)
Other	(62)	0
Net cash provided by operating activities	321,084	72,287
CASH FLOWS FROM INVESTING ACTIVITIES:
Maturities of held-to-maturity investments	84,000	88,000
Purchases of held-to-maturity investments	(349,155)	(49,451)
Purchases of property and equipment	(1,380)	(2,280)
Other	15	0
Net cash (used in) provided by investing activities	(266,520)	36,269
CASH FLOWS FROM FINANCING ACTIVITIES:
Proceeds from issuance of common stock, net of placement agent commissions	39,192	0
Payments of common stock offering costs	0	(50)
Proceeds from exercises of stock options and issuances under Employee Stock Purchase Plan	588	196
Other	(12)	0
Net cash provided by financing activities	39,768	146
NET INCREASE IN CASH, CASH EQUIVALENTS, AND RESTRICTED CASH EQUIVALENTS	94,332	108,702
CASH, CASH EQUIVALENTS, AND RESTRICTED CASH EQUIVALENTS – Beginning of period	156,710	54,983
CASH, CASH EQUIVALENTS, AND RESTRICTED CASH EQUIVALENTS – End of period	251,042	163,685
NONCASH OPERATING ACTIVITIES
Right-of-use assets acquired through operating leases	1,101	667
NONCASH INVESTING ACTIVITIES
Property and equipment purchases included in accounts payable and accrued expenses	259	863
NONCASH FINANCING ACTIVITIES
Common stock offering costs included in accounts payable or accrued expenses	104	0
Right-of-use assets acquired through financing leases	48	0
Total cash, cash equivalents, and restricted cash equivalents shown in the condensed consolidated statements of cash flows	$ 251,042	$ 163,685