Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Operating activities:
Net loss	$ (27,379)	$ (16,443)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	9,407	4,775
Depreciation expense	3,831	4,422
Net amortization of premiums and discounts on investments	(1,107)	1,172
Loss on disposal of property and equipment	0	75
Changes in operating assets and liabilities:
Taxes receivable	16,737	0
Prepaid expenses and other current assets	873	(17,416)
Other non-current assets	0	(266)
Accounts payable	562	373
Accrued expenses and other current liabilities	(1,443)	4,120
Deferred revenue—related party	(65,201)	(71,644)
Deferred rent	107	(156)
Net cash used in operating activities	(63,613)	(90,988)
Investing activities:
Purchases of investments	(252,918)	(179,874)
Proceeds from maturities of investments	336,694	141,322
Proceeds from sales of investments	3,997	15,638
Purchases of property and equipment	(1,359)	(15,107)
Net cash provided by (used in) investing activities	86,414	(38,021)
Financing activities:
Proceeds from initial public offering of common stock, net of issuance costs	0	107,008
Proceeds from exercise of stock options	1,546	462
Net cash provided by financing activities	1,546	107,470
Net increase (decrease) in cash, cash equivalents and restricted cash	24,347	(21,539)
Cash, cash equivalents and restricted cash, beginning of period	24,829	46,368
Cash, cash equivalents and restricted cash, end of period	49,176	24,829
Non-cash investing and financing activities:
Purchases of property and equipment in accounts payable and accrued expenses	31	170
Supplemental cash flow information:
Cash paid for income taxes	$ 0	$ 16,750