Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Cash flows from operating activities:
Net loss	$ (55,706)	$ (53,492)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	8,570	8,781
Depreciation and amortization expense	3,895	3,490
Non-cash interest expense		2
Accretion of discount on investments	(132)	(116)
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(376)	287
Deferred revenue	(7,863)	(6,029)
Accounts payable	(1,537)	(3,651)
Accrued expenses and other current liabilities	712	(577)
Net cash (used in) operating activities	(52,437)	(51,305)
Cash flows from investing activities:
Purchases of property and equipment	(1,655)	(3,527)
Purchases of investments	(15,456)	(43,795)
Sales and maturities of investments	91,495	77,807
Net cash provided by investing activities	74,384	30,485
Cash flows from financing activities:
Proceeds from exercise of stock options	96	36
Proceeds from issuance of common stock and restricted common stock	26
Payments of employee tax obligations related to vesting of restricted stock units	(197)
Net cash (used in) provided by financing activities	(75)	36
Net increase (decrease) in cash and cash equivalents	21,872	(20,784)
Cash, cash equivalents and restricted cash at beginning of period	37,601	55,939
Cash, cash equivalents and restricted cash at end of period	59,473	35,155
Supplemental disclosure of non-cash investing and financing activities:
Property and equipment purchases included in accounts payable and accrued expenses	$ 90	$ 140