Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash flows from operating activities
Net loss	$ (25,621)	$ (31,553)	$ (41,007)
Adjustments to reconcile net loss to net cash provided by (used in) operating activities
Depreciation and amortization	6,101	5,988	6,486
Amortization of premiums on investments	35	38	87
(Gain) loss on disposal of property and equipment	(4)	25	(9)
Stock-based compensation	16,070	10,459	7,815
Accretion of capitalized loan fees	18	136	62
Loss on extinguishment of debt		722
Remeasurement of preferred stock warrant liability		202	57
Foreign exchange loss	25	412
Change in operating assets and liabilities
Accounts receivable	(10,784)	(5,765)	(6,193)
Prepaid expenses and other assets	3,338	(3,606)	(970)
Accounts payable	2,242	278	666
Accrued expenses	(428)	1,626	2,095
Deferred revenue	19,163	16,908	19,936
Deferred rent	(799)	136	384
Net cash provided by (used in) operating activities	9,356	(3,994)	(10,591)
Cash flows from investing activities
Purchases of property and equipment	(3,603)	(5,402)	(7,643)
Proceeds from sales of equipment	24
Proceeds from maturities of investments	50,457	6,245	16,839
Purchases of investments	(69,222)	(75,303)	(2,715)
Payments for security deposits	(62)	(184)	177
Net cash (used in) provided by investing activities	(22,406)	(74,644)	6,658
Cash flows from financing activities
Proceeds from exercises of common stock options	21,868	1,333	2,670
Proceeds from purchases of stock under employee stock purchase plan	4,381
Proceeds from initial public offering, net of underwriting discounts		102,672
Payment of initial public offering costs	(243)	(2,362)	(1,092)
Proceeds from long-term debt		20,000
Principal payments on long-term debt		(20,000)
Payment of debt prepayment fees		(200)
Principal payments on capital lease obligations	(43)	(43)	(45)
Payment of capitalized loan fees		(248)	(78)
Net cash provided by financing activities	25,963	101,152	1,455
Foreign currency effect on cash, cash equivalents and restricted cash	149	(263)	(37)
Net increase (decrease) in cash, cash equivalents and restricted cash	13,062	22,251	(2,515)
Cash, cash equivalents and restricted cash
Beginning of period	42,007	19,756	22,271
End of period	55,069	42,007	19,756
Supplemental disclosures
Cash paid for interest	1	771	13
Purchases under capital lease obligations			102
Property and equipment additions in accounts payable and accrued expenses	$ 453	639	234
Leasehold improvements paid directly by lessor		356
Non-cash warrants issued as debt issuance costs		285	90
Initial public offering costs in accounts payable and accrued expenses		244	$ 881
Non-cash preferred stock warrant exercise		$ 616