Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Cash flows from operating activities:
Net loss	$ (77,655)	$ (60,427)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	12,887	13,150
Depreciation and amortization expense	5,869	5,356
Non-cash interest expense		3
Accretion of discount on investments	(199)	(166)
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(1,461)	(220)
Deferred revenue	(16,328)	(9,044)
Accounts payable	106	(2,502)
Accrued expenses and other current liabilities	1,261	(1,065)
Net cash (used in) operating activities	(75,520)	(54,915)
Cash flows from investing activities:
Purchases of property and equipment	(2,133)	(3,991)
Purchases of investments	(21,832)	(75,728)
Sales and maturities of investments	118,887	126,125
Net cash provided by investing activities	94,922	46,406
Cash flows from financing activities:
Proceeds from exercise of stock options	136	108
Proceeds from issuance of common stock and restricted common stock	26
Payments of employee tax obligations related to vesting of restricted stock units	(196)
Issuance of common stock under ESPP plan	257
Net cash provided by financing activities	223	108
Net increase (decrease) in cash and cash equivalents	19,625	(8,401)
Cash, cash equivalents and restricted cash at beginning of period	37,601	55,939
Cash, cash equivalents and restricted cash at end of period	57,226	47,538
Supplemental disclosure of non-cash investing and financing activities:
Property and equipment purchases included in accounts payable and accrued expenses	$ 242	$ 453