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	$ (47,973)	$ (24,350)	$ (20,014)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	869	829	714
Issuance of common stock for technology	1,411
Stock-based compensation	4,856	1,791	155
Amortization of debt discount	297	426	211
Change in fair value of preferred stock warrant liability	(29)	(36)	28
Amortization of investment premium	789	43
Deferred rent	(295)	(277)	(254)
Changes in operating assets and liabilities
Prepaid expenses and other assets	(666)	(1,043)	323
Accounts payable and accrued expenses	3,944	(207)	1,526
Net cash used in operating activities	(36,797)	(22,824)	(17,311)
Cash flows from investing activities:
Purchase of property and equipment	(664)	(249)	(644)
Purchases of investment securities	(109,445)	(5,397)
Maturities of investment securities	38,115	3,400
Net cash used in investing activities	(71,994)	(2,246)	(644)
Cash flows from financing activities:
Issuance of preferred stock for cash, net of issuance costs	75,648		38,660
Issuance of common stock through initial public offering, net of offering costs	76,902
Costs paid in connection with initial public offering		(999)
Proceeds from issuance of common stock through option exercises	604	72	77
Proceeds from notes payable to bank		5,000	5,000
Repayments on notes payable to bank	(3,237)	(1,561)	(2,500)
Proceeds from convertible debt			10,000
Repayment of convertible debt	(2,000)		(500)
Net cash provided by financing activities	147,917	2,512	50,737
Net change in cash and cash equivalents	39,126	(22,558)	32,782
Cash and cash equivalents at beginning of the period	13,899	36,457	3,675
Cash and cash equivalents at the end of the period	53,025	13,899	36,457
Supplemental disclosure of cash flow information:
Interest paid	925	415	254
Supplemental schedule of noncash investing and financing activities:
Issuance of warrants in connection with long-term debt		148	137
Changes in share repurchase liability	$ (120)	$ 13	(3)
Capital contribution related to reversal of historical accretion of redeemable convertible preferred stock			17,528
Conversion of convertible debt and accrued interest			$ 9,537