Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Cash flows from operating activities:
Net loss	$ (27,919)	$ (25,474)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	4,236	4,279
Depreciation and amortization expense	1,941	1,730
Non-cash interest expense		3
Accretion of discount on investments	(50)	(59)
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	835	(62)
Deferred revenue	(3,592)	(3,015)
Accounts payable	(1,426)	(1,916)
Accrued expenses and other current liabilities	(788)	(1,135)
Net cash used in operating activities	(26,763)	(25,649)
Cash flows from investing activities:
Purchases of property and equipment	(953)	(2,247)
Purchases of investments	(5,270)	(22,513)
Sales and maturities of investments	44,257	37,751
Net cash provided by investing activities	38,034	12,991
Cash flows from financing activities:
Proceeds from exercise of stock options and common stock warrants	32	19
Payments of employee tax obligations related to vesting of restricted stock units	(197)
Net cash (used in) provided by financing activities	(165)	19
Net increase (decrease) in cash, cash equivalents and restricted cash	11,106	(12,639)
Cash, cash equivalents and restricted cash at beginning of period	37,601	55,939
Cash, cash equivalents and restricted cash at end of period	48,707	43,300
Supplemental disclosure of non-cash investing and financing activities:
Property and equipment purchases included in accounts payable and accrued expenses	$ 393	$ 349