Consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014
Cash flows from operating activities:
Net loss	$ (35,146)	$ (8,527)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	6,907	700
Depreciation and amortization expense	394	113
Loss from revaluation of preferred stock warrant liability	7	504
Licensing fees paid in common stock warrant		317
Non-cash interest expense	269	64
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(2,962)	(238)
Accounts payable	532	519
Accrued expenses and other current liabilities	724	305
Net cash used in operating activities	(29,275)	(6,243)
Cash flows from investing activities:
Purchases of property and equipment	(3,126)	(627)
Purchases of investments	(197,100)
Maturities of investments	47,295
Changes in restricted cash		(102)
Net cash used in investing activities	(152,931)	(729)
Cash flows from financing activities:
Proceeds from issuance of convertible preferred stock, net of issuance costs	(24)	10,558
Proceeds from issuance of notes payable		2,000
Proceeds from exercise of stock options and common stock warrants	260	5
Proceeds from issuance of common stock upon completion of initial public offering	143,015
Repayment of notes payable	(2,600)	(100)
Payments of initial public offering costs	(2,928)	(25)
Net cash provided by financing activities	137,723	12,438
Net (decrease) increase in cash and cash equivalents	(44,483)	5,466
Cash and cash equivalents at beginning of period	114,185	1,654
Cash and cash equivalents at end of period	69,702	7,120
Supplemental disclosure of cash flow information:
Cash paid for interest	162	93
Supplemental disclosure of non-cash investing and financing activities:
Conversion of convertible preferred stock into common stock upon listing of the Company’s common stock on the NASDAQ	136,053
Accretion of convertible preferred stock to redemption value		1,019
Deferred offering costs included in accounts payable and accrued expenses		$ 97
Property and equipment purchases included in accounts payable and accrued expenses	$ 648