Significant Accounting Policies - Impacts on Statements of Cash Flows from the Adoption of ASC 606 (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net loss	$ (10,797)	$ (14,650)	$ (10,051)	$ (15,216)	$ (13,903)	$ (15,028)	$ (10,967)	$ (9,553)	$ (50,714)	$ (49,451)	$ (31,007)
Depreciation and amortization									4,742	2,021	886
Loss (gain) on disposal of equipment									146	(4)	0
Bad debt expense									99	211	62
Deferred income taxes									(334)	(218)	(251)
Stock-based compensation									16,443	16,054	12,976
Accounts receivable									7,432	(23,332)	(9,716)
Prepaid expenses and other assets									8,972	(1,025)	(4,162)
Deferred commissions									(9,319)	(7,615)	(3,487)
Accounts payable and accrued expenses									(4,039)	7,461	4,128
Accrued compensation and related benefits									(3,072)	(3)	2,365
Other current liabilities									1,318	1,823	383
Deferred revenue									12,573	23,023	18,344
Operating lease liabilities									6,827
Deferred rent, non-current									0	(266)	(374)
Net cash used in operating activities									(8,926)	(31,321)	(9,128)
Purchases of property and equipment									(32,421)	(7,014)	(433)
Proceeds from sale of equipment									0	4	0
Net cash used in investing activities									(32,421)	(7,010)	(433)
Proceeds from public offering, net of underwriting discounts									101,653	58,258	0
Payment of costs related to public offerings									(350)	(429)	(2,424)
Proceeds from exercise of common stock options									4,899	3,133	1,108
Principal payments on finance lease obligations									(653)
Net cash provided by financing activities									105,549	60,962	50,948
Effect of foreign exchange rate changes on cash and cash equivalents									623	(1,459)	1,228
Net increase in cash and cash equivalents									64,825	21,172	42,615
Cash and cash equivalents, beginning of period				94,930				$ 73,758	94,930	73,758	31,143
Cash and cash equivalents, end of period	159,755				94,930				159,755	94,930	$ 73,758
Calculated under Revenue Guidance in Effect before Topic 606
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net loss									(49,391)
Depreciation and amortization									4,742
Loss (gain) on disposal of equipment									146
Bad debt expense									99
Deferred income taxes									(334)
Stock-based compensation									16,443
Accounts receivable									7,432
Prepaid expenses and other assets									4,659
Deferred commissions									(4,694)
Accounts payable and accrued expenses									(4,039)
Accrued compensation and related benefits									(3,072)
Other current liabilities									1,318
Deferred revenue									10,856
Operating lease liabilities									6,827
Deferred rent, non-current									0
Net cash used in operating activities									(9,008)
Purchases of property and equipment									(32,421)
Proceeds from sale of equipment									0
Net cash used in investing activities									(32,421)
Proceeds from public offering, net of underwriting discounts									101,653
Payment of costs related to public offerings									(350)
Proceeds from exercise of common stock options									4,899
Principal payments on finance lease obligations									(653)
Net cash provided by financing activities									105,549
Effect of foreign exchange rate changes on cash and cash equivalents									705
Net increase in cash and cash equivalents									64,825
Cash and cash equivalents, beginning of period				94,930					94,930
Cash and cash equivalents, end of period	159,755				94,930				159,755	94,930
Accounting Standards Update 2014-09 | Difference between Revenue Guidance in Effect before and after Topic 606
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net loss									(1,323)
Depreciation and amortization									0
Loss (gain) on disposal of equipment									0
Bad debt expense									0
Deferred income taxes									0
Stock-based compensation									0
Accounts receivable									0
Prepaid expenses and other assets									4,313
Deferred commissions									(4,625)
Accounts payable and accrued expenses									0
Accrued compensation and related benefits									0
Other current liabilities									0
Deferred revenue									1,717
Operating lease liabilities									0
Deferred rent, non-current									0
Net cash used in operating activities									82
Purchases of property and equipment									0
Proceeds from sale of equipment									0
Net cash used in investing activities									0
Proceeds from public offering, net of underwriting discounts									0
Payment of costs related to public offerings									0
Proceeds from exercise of common stock options									0
Principal payments on finance lease obligations									0
Net cash provided by financing activities									0
Effect of foreign exchange rate changes on cash and cash equivalents									(82)
Net increase in cash and cash equivalents									0
Cash and cash equivalents, beginning of period				$ 0					0
Cash and cash equivalents, end of period	$ 0				$ 0				$ 0	$ 0