Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Cash flows from operating activities:
Net loss	$ (59,668)	$ (26,458)	$ (10,819)
Adjustments to reconcile net loss to net cash used in operating activities:
Amortization of discount on investments	51	660	0
Depreciation and amortization	1,630	166	35
Stock-based compensation	2,004	1,291	515
Accretion of asset retirement obligation	106	136	0
Change in fair value of contingent acquisition consideration payable	102	131	0
Other	71	0	(9)
Changes in operating assets and liabilities:
Additions to restricted cash	(90)	(3,610)	0
Prepaid expenses and other current assets	1,085	(3,188)	(300)
Other assets	27	(195)	(36)
Accounts payable	(671)	628	857
Accrued liabilities	4,815	2,634	1,683
Deferred rent	1,522	290	5
Net cash used in operating activities	(49,016)	(27,515)	(8,069)
Cash flows from investing activities:
Cash acquired from acquisition	0	142	0
Proceeds from sales of property and equipment	16	0	0
Purchases of property and equipment	(16,651)	(1,686)	(125)
Proceeds from sales and maturities of marketable securities	58,624	32,792	0
Purchases of marketable securities	(104,037)	(40,124)	(16,539)
Net cash used in investing activities	(62,048)	(8,876)	(16,664)
Cash flows from financing activities:
Proceeds from exercise of stock options	157	70	0
Proceeds from issuance of common stock from initial public offering, net of offering costs	75,208	0	0
Net cash provided by financing activities	75,365	62,850	57,386
Net (decrease) increase in cash and cash equivalents	(35,699)	26,459	32,653
Cash and cash equivalents at beginning of period	72,058	45,599	12,946
Cash and cash equivalents at end of period	36,359	72,058	45,599
Noncash investing and financing activities:
Change in accounts payable, accrued liabilities and facility lease obligations related to property and equipment purchases	927	1,140	65
Conversion of redeemable convertible preferred stock to common stock at closing of initial public offering	135,750	0	0
Deferred financing costs for initial public offering	2,333	0	0
Preferred stock issuance costs	0	140	(120)
Issuance of common stock in exchange for license	0	128	0
Convertible Preferred Stock, Series A
Cash flows from financing activities:
Proceeds from issuance of convertible preferred stock, net of issuance costs	0	0	14,998
Convertible Preferred Stock, Series B
Cash flows from financing activities:
Proceeds from issuance of convertible preferred stock, net of issuance costs	0	0	42,388
Convertible Preferred Stock, Series C
Cash flows from financing activities:
Proceeds from issuance of convertible preferred stock, net of issuance costs	0	62,780	0
Cardiogen Sciences, Inc
Noncash investing and financing activities:
Net assets acquired and stock issued for the Cardiogen acquisition	$ 0	$ 8,513	$ 0