Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Cash flows from operating activities:
Net loss	$ (24,333)	$ (27,919)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	2,065	4,236
Depreciation and amortization expense	2,006	1,941
Amortization of operating lease asset	535
Accretion of discount on investments		(50)
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(643)	835
Deferred revenue	(208)	(3,592)
Accounts payable	(2,474)	(1,426)
Operating lease liabilities	(1,024)
Accrued expenses and other current liabilities	(1,650)	(788)
Accounts receivable	(6,667)
Net cash (used in) operating activities	(32,393)	(26,763)
Cash flows from investing activities:
Purchases of property and equipment	(306)	(953)
Purchases of investments		(5,270)
Sales and maturities of investments		44,257
Net cash (used in) provided by investing activities	(306)	38,034
Cash flows from financing activities:
Proceeds from exercise of stock options	120	32
Proceeds from issuance of common stock and restricted common stock	153
Payments of employee tax obligations related to vesting of restricted stock units		(197)
Issuance of common stock under ESPP plan	207
Net cash provided by (used in) financing activities	480	(165)
Net (decrease) increase in cash and cash equivalents	(32,219)	11,106
Cash, cash equivalents and restricted cash at beginning of period	85,933	37,601
Cash, cash equivalents and restricted cash at end of period	53,714	48,707
Supplemental disclosure of non-cash investing and financing activities:
Property and equipment purchases included in accounts payable and accrued expenses	$ 134	$ 393