STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash flows from operating activities:
Net loss	$ (43,099)	$ (58,900)	$ (35,374)
Adjustments to reconcile net loss to net cash (used) provided by operating activities:
Loss/(gain) on disposal of property and equipment	17	(47)	25
Depreciation and amortization	1,645	1,733	1,206
Amortization of debt discount			80
Accretion of discount on short-term investments	371	1,662	1,186
Stock-based compensation expense	11,287	10,095	3,986
Issuance of common stock in connection with services		159
Non-cash acquisition of in-process research and development asset charged to expense	10,700
Deferred income taxes	(513)	6	8
Changes in operating assets and liabilities
Accounts receivable	(7,980)	(1,787)	1,131
Related party accounts receivable	154	218
Prepaid expenses and other current assets	(456)	(1,597)	(1,491)
Other assets	1,618	(2,609)	128
Accounts payable	(2,441)	1,765	2,944
Deferred revenue	189,913	43,317	(6,130)
Accrued liabilities and other liabilities	9,157	3,953	3,170
Net cash provided by/(used in) operating activities	170,373	(2,032)	(27,415)
Cash flows from investing activities:
Purchases of property and equipment	(1,559)	(2,176)	(1,594)
Proceeds from sales of assets		52
Purchases of investments	(218,707)	(121,517)	(250,901)
Maturities of investments	99,000	169,500	122,750
Increase in restricted cash			(817)
Net cash (used in)/provided by investing activities	(121,266)	45,859	(130,562)
Cash flows from financing activities:
Proceeds from issuance of redeemable convertible preferred stock, net of issuance costs			74,430
Proceeds from issuance of common stock, net of issuance costs	19,957
Proceeds from initial public offering, net of issuance costs			81,777
Repayment of notes payable			(3,067)
Payment of deferred offering costs		(12)
Net cash provided by financing activities	23,796	996	153,403
Proceeds from employee stock purchases and exercise of stock options	3,839	930	263
Proceeds from stockholder notes		78
Net increase/(decrease) in cash and cash equivalents	72,903	44,823	(4,574)
Cash and cash equivalents, beginning of year	104,645	59,822	64,396
Cash and cash equivalents, end of year	177,548	104,645	59,822
Supplemental disclosures of noncash investing and financing items:
Purchases of property and equipment in accounts payable and accrued liabilities	$ 361	$ 473	100
Accretion to redemption value and cumulative dividends on preferred stock			6,705
Convertible preferred stock liability recorded in connection with redeemable convertible preferred stock			1,509
Stock issuance costs in accounts payable and accrued liabilities			13
Convertible Preferred Stock Liability
Adjustments to reconcile net loss to net cash (used) provided by operating activities:
Change in fair value of liability			1,114
Convertible Preferred Stock Warrant Liability
Adjustments to reconcile net loss to net cash (used) provided by operating activities:
Change in fair value of liability			$ 602