Consolidated Statements of Cash Flows - USD ($) $ in Thousands	8 Months Ended	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2016	Dec. 31, 2015
CASH FLOWS FROM OPERATING ACTIVITIES:
Net loss	$ (9,539)	$ (31,634)	$ (12,397)
Adjustments to reconcile net loss to net cash provided by (used in) operating activities:
Acquired in-process research and development	6,055
Depreciation and amortization	3	1,104	328
Loss on disposal of property and equipment		13	9
Equity-based compensation	308	6,715	1,308
Benefit from intraperiod tax allocation			(1,012)
Changes in operating assets and liabilities:
Accounts receivable		(5,454)
Prepaid expenses and other current assets	(285)	(979)	(525)
Accounts payable	163	1,784	335
Accrued expenses	1,056	3,050	805
Deferred revenue		67,975	9,312
Other assets	(256)	(6,435)	(76)
Other long-term liabilities	173	(30)	150
Net cash provided by (used in) operating activities	(2,322)	36,109	(1,763)
CASH FLOWS FROM INVESTING ACTIVITIES:
Purchases of property and equipment	(275)	(6,165)	(2,554)
Acquisition of in-process research and development	(300)
Net cash used in investing activities	(575)	(6,165)	(2,554)
CASH FLOWS FROM FINANCING ACTIVITIES:
Proceeds from sale of Class A-1 and A-2 preferred units and Series B preferred stock	12,651		74,661
Payments to acquire in-process research and development		(600)	(1,100)
Payment of preferred unit and preferred stock issuance costs	(258)	(100)	(2,671)
Proceeds from sale of common units	349
Proceeds from common stock offering		170,507
Proceeds from options exercised		2
Issuance of shares through employee stock purchase plan		259
Payment of common stock offering costs		(2,764)	(602)
Net cash provided by financing activities	12,742	167,304	70,288
Net increase in cash and cash equivalents	9,845	197,248	65,971
Cash and cash equivalents, beginning of period		75,816	9,845
Cash and cash equivalents, end of period	9,845	273,064	75,816
SUPPLEMENTAL DISCLOSURES OF CASH FLOW INFORMATION:
Purchases of property and equipment unpaid at period end	36	3,090	219
Acquisition of in-process research and development unpaid at period end	1,700		600
Noncash portion of acquired in-process research and development	$ 4,055
Financing costs incurred but unpaid at period end			$ 970
Conversion of convertible preferred stock to common stock		$ 88,557