Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Cash flows from operating activities:
Net loss	$ (54,780)	$ (16,709)	$ (6,102)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	9,694	2,068	209
Depreciation and amortization expense	728	190	88
Loss from revaluation of preferred stock warrant liability	7	1,418	8
Licensing fees paid in common stock warrant		317
Other non-cash expense	367	81	19
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(2,470)	(7)	(21)
Accounts payable	2,682	810	281
Accrued expenses and other liabilities	2,928	1,474	197
Net cash used in operating activities	(40,844)	(10,358)	(5,321)
Cash flows from investing activities:
Purchases of property and equipment	(4,362)	(1,001)	(174)
Purchases of Short-term investments	(267,761)
Maturities of Short-term investments	136,390
Changes in restricted cash	(1,400)	(102)	(10)
Net cash used in investing activities	(137,133)	(1,103)	(184)
Cash flows from financing activities:
Proceeds from issuance of convertible preferred stock, net of issuance costs	(24)	123,203
Proceeds from issuance of notes payable and preferred stock warrant, net of issuance costs		2,000	944
Proceeds from exercise of stock options and warrants	262	5
Proceeds from Issuance of common stock in connection with IPO	143,015
Proceeds from issuance of common stock and restricted common stock		5
Repayment of notes payable	(2,600)	(400)
Payments of initial public offering costs	(2,928)	(821)
Net cash provided by financing activities	137,725	123,992	944
Net increase (decrease) in cash and cash equivalents	(40,252)	112,531	(4,561)
Cash and cash equivalents at beginning of year	114,185	1,654	6,215
Cash and cash equivalents at end of year	73,933	114,185	1,654
Supplemental disclosure of cash flow information:
Cash paid for interest	163	122	20
Supplemental disclosure of non-cash investing and financing activities:
Accretion of convertible preferred stock to redemption value		1,291	875
Issuance of preferred stock warrant in connection with notes payable			$ 156
Deferred offering costs included in accounts payable		863
Property and equipment purchases included in accounts payable and accrued expenses	$ 2,953	$ 101