Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Operating activities
Net loss	$ (77,480)	$ (83,189)	$ (66,742)
Adjustments to reconcile net loss to net cash used in operating activities
Depreciation and amortization	282	193	124
Amortization of deferred financing costs and debt discount		94	226
Accretion of final interest payment on term loans		45	115
Stock-based compensation expense	13,145	11,616	5,230
Loss from disposal of property and equipment		2	2
Changes in operating assets and liabilities:
Accounts receivable	2,362	(693)	(1,752)
Prepaid expenses and other assets	(2,870)	(1,514)	(1,199)
Accounts payable	(302)	(1,249)	2,224
Accrued expenses and other liabilities	751	(2,663)	3,983
Deferred revenue	346	651	166
Net cash used in operating activities	(63,766)	(76,707)	(57,623)
Investing activities
Purchases of property and equipment	(393)	(838)	(191)
Net cash used in investing activities	(393)	(838)	(191)
Financing activities
Proceeds from sale of common stock, net of underwriter discounts and issuance costs		162,151	80,766
Repayment of term loan payable		(4,646)	(6,126)
Proceeds from issuance of stock under stock plans	333	4,910	1,504
Net cash provided by financing activities	333	162,415	76,144
Net increase (decrease) in cash and cash equivalents	(63,826)	84,870	18,330
Cash and cash equivalents at beginning of period	205,912	121,042	102,712
Cash and cash equivalents at end of period	$ 142,086	205,912	121,042
Supplemental cash flow information
Cash paid for interest		$ 363	$ 945