Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Operating activities:
Net income (loss)	$ 56,823	$ (27,379)
Adjustments to reconcile net income (loss) to net cash used in operating activities:
Stock-based compensation expense	9,609	9,407
Depreciation expense	3,851	3,831
Net amortization of premiums and discounts on investments	(1,480)	(1,107)
Changes in operating assets and liabilities:
Taxes receivable	0	16,737
Prepaid expenses and other current assets	(1,375)	873
Other non-current assets	(728)	0
Accounts payable	(944)	562
Accrued expenses and other current liabilities	1,983	(1,443)
Deferred revenue—related party	(97,872)	(65,201)
Other liabilities	4	107
Net cash used in operating activities	(30,129)	(63,613)
Investing activities:
Purchases of investments	(188,999)	(252,918)
Proceeds from maturities of investments	221,366	336,694
Proceeds from sales of investments	0	3,997
Purchases of property and equipment	(985)	(1,359)
Net cash provided by investing activities	31,382	86,414
Financing activities:
Proceeds from at the market offering, net of issuance costs	3,645	0
Proceeds from exercise of stock options	437	1,546
Net cash provided by financing activities	4,082	1,546
Net increase in cash, cash equivalents and restricted cash	5,335	24,347
Cash, cash equivalents and restricted cash, beginning of period	49,176	24,829
Cash, cash equivalents and restricted cash, end of period	54,511	49,176
Non-cash investing and financing activities:
Purchases of property and equipment in accounts payable and accrued expenses	29	31
Issuance costs in accounts payable and accrued expenses	134	0
Supplemental cash flow information:
Cash paid for lease liabilities	4,270	0
Cash paid for income taxes	$ 101	$ 0