Consolidated Statements of Cash Flows - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Operating activities
Net (loss) income		$ (55,489)	$ (37,966)	$ 17,859
Adjustments to reconcile net (loss) income to net cash (used in) provided by operating activities:
Stock-based compensation		11,638	10,833	8,748
Depreciation of property and equipment		550	512	418
Amortization of right-of-use assets		1,092	0	0
Noncash interest (income) expense, net		(1,499)	(1,071)	143
Changes in assets and liabilities:
Accounts receivable, other		(176)	0	0
Accounts receivable from related party		2,058	49,032	(20,885)
Prepaids and other current assets		719	(668)	(727)
Other assets		61	(31)	(80)
Accounts payable		188	(434)	729
Other liabilities		4,459	4,158	80
Deferred revenue from related party		(33,724)	(7,929)	(1,407)
Net cash (used in) provided by operating activities		(70,123)	16,436	4,878
Investing activities
Purchases of property and equipment, net		(790)	(838)	(723)
Purchases of investments		(211,973)	(192,480)	(133,845)
Sales of investments		4,967	0	0
Maturities of investments		195,270	140,250	150,170
Net cash (used in) provided by investing activities		(12,526)	(53,068)	15,602
Financing activities
Proceeds from issuance of common stock		73,276	0	0
Proceeds from exercise of stock options and employee stock purchase plan		22,857	10,467	3,269
Employees' tax withheld and paid for restricted stock units		(1,313)	(678)	(429)
Borrowings under credit facility agreement, net of issuance costs		0	14,911	4,676
Net cash provided by financing activities		94,820	24,700	7,516
Net increase (decrease) in cash, cash equivalents and restricted cash		12,171	(11,932)	27,996
Cash, cash equivalents and restricted cash at beginning of period		28,088	40,020	12,024
Cash, cash equivalents and restricted cash at end of period		40,259	28,088	40,020
Supplemental disclosures of cash flow information
Cash paid for interest		1,735	748	0
Right-of-use assets obtained in exchange for lease obligations	[1]	2,796	0	0
Purchases of property and equipment, net recorded in accounts payable		$ 378	$ 0	$ 0

[1] Amounts for the year ended December 31, 2019 include the transition adjustment of $1,301 for the adoption of Topic 842.