Significant Accounting Policies - Impacts on Statements of Operations from the Adoption of ASC 606 (Details) - USD ($) $ / shares in Units, $ 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]
Total revenue	$ 68,618	$ 66,155	$ 65,275	$ 60,304	$ 60,216	$ 54,948	$ 59,883	$ 51,696	$ 260,352	$ 226,743	$ 176,737
Total cost of revenue	22,773	23,951	23,051	24,066	22,210	20,092	21,574	21,049	93,841	84,925	64,597
Gross profit	45,845	42,204	42,224	36,238	38,006	34,856	38,309	30,647	166,511	141,818	112,140
Sales and marketing	31,254	27,603	29,992	28,591	30,177	25,467	27,384	22,964	117,440	105,992	81,966
Research and development	15,625	15,697	12,765	13,956	12,332	11,737	10,785	9,870	58,043	44,724	34,835
General and administrative	12,028	11,191	9,261	9,016	8,799	12,537	8,425	8,060	41,496	37,821	27,150
Total operating expenses	58,907	54,491	52,018	51,563	51,308	49,741	46,594	40,894	216,979	188,537	143,951
Operating loss	(13,062)	(12,287)	(9,794)	(15,325)	(13,302)	(14,885)	(8,285)	(10,247)	(50,468)	(46,719)	(31,811)
Other (income) expense, net	(2,822)	2,262	(79)	(302)	510	110	2,593	(918)	(941)	2,295	(2,038)
Interest expense	131	96	69	71	64	67	54	13	367	198	473
Total other (income) expense	(2,691)	2,358	(10)	(231)	574	177	2,647	(905)	(574)	2,493	(1,565)
Loss before income taxes	(10,371)	(14,645)	(9,784)	(15,094)	(13,876)	(15,062)	(10,932)	(9,342)	(49,894)	(49,212)	(30,246)
Income tax expense	426	5	267	122	27	(34)	35	211	820	239	761
Net loss	(10,797)	(14,650)	(10,051)	(15,216)	(13,903)	(15,028)	(10,967)	(9,553)	$ (50,714)	$ (49,451)	$ (31,007)
Basic and diluted (in usd per share)									$ (0.77)	$ (0.80)	$ (0.63)
Subscriptions
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total revenue	42,108	37,774	36,860	34,557	35,108	30,905	33,047	26,952	$ 151,299	$ 126,012	$ 91,514
Total cost of revenue	4,993	4,484	4,036	3,585	3,284	3,261	2,824	2,628	17,098	11,997	9,379
Professional services
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total revenue	26,510	28,381	28,415	25,747	25,108	24,043	26,836	24,744	109,053	100,731	85,223
Total cost of revenue	17,780	19,467	19,015	20,481	$ 18,926	$ 16,831	$ 18,750	$ 18,421	76,743	$ 72,928	$ 55,218
Calculated under Revenue Guidance in Effect before Topic 606
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total revenue	70,470	69,387	66,911	59,579					266,347
Total cost of revenue									93,841
Gross profit									172,506
Sales and marketing									122,065
Research and development									58,043
General and administrative									41,496
Total operating expenses									221,604
Operating loss									(49,098)
Other (income) expense, net									(894)
Interest expense									367
Total other (income) expense									(527)
Loss before income taxes									(48,571)
Income tax expense									820
Net loss									$ (49,391)
Basic and diluted (in usd per share)									$ (0.75)
Calculated under Revenue Guidance in Effect before Topic 606 | Subscriptions
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total revenue									$ 160,085
Total cost of revenue									17,098
Calculated under Revenue Guidance in Effect before Topic 606 | Professional services
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total revenue	26,152	27,788	27,652	24,670					106,262
Total cost of revenue									76,743
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]
Total revenue	(1,852)	(3,232)	(1,636)	725					(5,995)
Total cost of revenue									0
Gross profit									(5,995)
Sales and marketing									(4,625)
Research and development									0
General and administrative									0
Total operating expenses									(4,625)
Operating loss									(1,370)
Other (income) expense, net									(47)
Interest expense									0
Total other (income) expense									(47)
Loss before income taxes									(1,323)
Income tax expense									0
Net loss									$ (1,323)
Basic and diluted (in usd per share)									$ (0.02)
Accounting Standards Update 2014-09 | Difference between Revenue Guidance in Effect before and after Topic 606 | Subscriptions
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total revenue									$ (8,786)
Total cost of revenue									0
Accounting Standards Update 2014-09 | Difference between Revenue Guidance in Effect before and after Topic 606 | Professional services
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total revenue	$ 358	$ 593	$ 763	$ 1,077					2,791
Total cost of revenue									$ 0