Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
CASH FLOWS FROM OPERATING ACTIVITIES
Consolidated net loss	$ (172,871)	$ (173,482)	$ (118,476)
Adjustments to reconcile consolidated net loss to net cash used in operating activities:
Non-cash stock compensation expense	11,335	12,886	15,945
Shares issued in connection with in-licensing agreement	100	4,000	0
Depreciation and amortization	100	88	82
Amortization of premium on investment securities			61
Amortization of discount on investment securities	(257)	(119)
Amortization of debt issuance costs	772	0	0
Amortization of leasehold interest	182	135	125
Non-cash change in lease liability and right-of-use asset	2,519	0	0
Change in fair value of notes payable and accrued interest	124	(61)	59
Changes in assets and liabilities:
Decrease (increase) in other current assets	1,395	(1,638)	(2,665)
(Increase) decrease in accrued interest receivable	(11)	14	(25)
Decrease in other assets	0	0	162
(Decrease) increase in accounts payable and accrued expenses	(4,795)	10,957	11,020
Decrease in lease liabilities	(1,548)	0	0
Increase in interest payable	1,491	0	0
Increase in other liabilities	28,810	18,350	0
Increase in deferred rent	0	97	104
Decrease in deferred revenue	(152)	(152)	(152)
Net cash used in operating activities	(132,806)	(128,925)	(93,760)
CASH FLOWS FROM INVESTING ACTIVITIES
Proceeds from maturity of short-term securities	29,250	32,500	19,800
Investment in held-to-maturity securities	(29,837)	(31,230)	(28,006)
Purchases of equipment	(131)	(90)	(2)
Net cash (used in) provided by investing activities	(718)	1,180	(8,208)
CASH FLOWS FROM FINANCING ACTIVITIES
Proceeds from sale of common stock, net	175,021	113,639	131,516
Proceeds from debt financings	29,987	0	0
Proceeds from the exercise of warrants	0	0	2,143
Financing costs paid	(795)	0	0
Net cash provided by financing activities	204,213	113,639	133,659
NET INCREASE (DECREASE) IN CASH, CASH EQUIVALENTS AND RESTRICTED CASH	70,689	(14,106)	31,691
CASH, CASH EQUIVALENTS AND RESTRICTED CASH AT BEGINNING OF YEAR	43,199	57,305	25,614
CASH, CASH EQUIVALENTS AND RESTRICTED CASH AT END OF YEAR	$ 113,888	$ 43,199	$ 57,305