Revenue - Impacts on financial statements (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	Jan. 01, 2018
Current assets
Accounts receivable, net of allowance	$ 62,405				$ 59,631				$ 62,405	$ 59,631
Contract asset	7,058				7,962				7,058	7,962
Other current assets	6,346				5,705				6,346	5,705
Total current assets	211,619				191,956				211,619	191,956
Deferred income taxes	7,863				5,601				7,863	5,601
Contract assets - non-current	3,565				3,316				3,565	3,316
Other assets	8,668				8,400				8,668	8,400
Total assets	384,570				352,826				384,570	352,826
Current liabilities
Deferred revenue	30,338				33,633				30,338	33,633
Short-term income taxes payable	7,711				6,905				7,711	6,905
Other accrued expenses	8,786				9,323				8,786	9,323
Total current liabilities	74,113				72,357				74,113	72,357
Deferred revenue - non-current	15,259				10,672				15,259	10,672
Deferred income taxes	4,623				2,661				4,623	2,661
Total liabilities	120,549				100,385				120,549	100,385
Stockholders' equity
Accumulated income	181,167				172,378				181,167	172,378
Total stockholders' equity	264,021				252,441				264,021	252,441
Total liabilities and stockholders' equity	384,570				352,826				384,570	352,826
Revenue
Total revenue	71,003	$ 79,725	$ 56,234	$ 47,608	64,799	$ 52,495	$ 49,554	$ 45,432	254,570	212,280	$ 193,291
Cost of goods sold
Total cost of goods sold									81,962	64,813	58,777
Gross profit	49,629	53,056	38,354	31,569	42,246	34,543	35,981	34,697	172,608	147,467	134,514
Operating costs
Sales and marketing									61,503	63,805	58,994
Total operating costs	43,735	35,937	40,565	37,096	38,145	37,657	38,592	33,049	157,333	147,443	128,322
Operating income	5,894	17,119	(2,211)	(5,527)	4,101	(3,114)	(2,611)	1,648	15,275	24	6,192
Income (loss) before taxes									15,495	3,553	8,381
Provision (benefit) for income taxes									6,706	(293)	30,780
Net income (loss)	$ 5,050	$ 11,872	$ (2,461)	$ (5,671)	$ 3,964	$ (908)	$ (1,002)	$ 1,792	$ 8,789	$ 3,846	$ (22,399)
Basic (in dollars per share)	$ 0.13	$ 0.30	$ (0.06)	$ (0.14)	$ 0.10	$ (0.02)	$ (0.03)	$ 0.04	$ 0.22	$ 0.10	$ (0.56)
Diluted (in dollars per share)	$ 0.13	$ 0.30	$ (0.06)	$ (0.14)	$ 0.10	$ (0.02)	$ (0.03)	$ 0.04	$ 0.22	$ 0.10	$ (0.56)
Adjustments
Current assets
Accounts receivable, net of allowance					$ (331)					$ (331)
Contract asset					(7,962)					(7,962)
Other current assets					(703)					(703)
Total current assets					(8,996)					(8,996)
Deferred income taxes					587					587
Contract assets - non-current					(3,316)					(3,316)
Other assets					(289)					(289)
Total assets					(12,014)					(12,014)
Current liabilities
Deferred revenue					3,019					3,019
Short-term income taxes payable					(1,209)					(1,209)
Other accrued expenses					(116)					(116)
Total current liabilities					1,694					1,694
Deferred revenue - non-current					3,391					3,391
Deferred income taxes					(424)					(424)
Total liabilities					4,661					4,661
Stockholders' equity
Accumulated income					(16,675)					(16,675)
Total stockholders' equity					(16,675)					(16,675)
Total liabilities and stockholders' equity					(12,014)					(12,014)
Revenue
Total revenue										(3,407)
Cost of goods sold
Total cost of goods sold										605
Gross profit										(4,012)
Operating costs
Sales and marketing										1,108
Total operating costs										1,108
Operating income										(5,120)
Income (loss) before taxes										(5,120)
Provision (benefit) for income taxes										(365)
Net income (loss)										(4,755)
Balances without the adoption of Topic 606
Current assets
Accounts receivable, net of allowance					59,300					59,300
Other current assets					5,002					5,002
Total current assets					182,960					182,960
Deferred income taxes					6,188					6,188
Other assets					8,111					8,111
Total assets					340,812					340,812
Current liabilities
Deferred revenue					36,652					36,652
Short-term income taxes payable					5,696					5,696
Other accrued expenses					9,207					9,207
Total current liabilities					74,051					74,051
Deferred revenue - non-current					14,063					14,063
Deferred income taxes					2,237					2,237
Total liabilities					105,046					105,046
Stockholders' equity
Accumulated income					155,703					155,703
Total stockholders' equity					235,766					235,766
Total liabilities and stockholders' equity					340,812					340,812
Revenue
Total revenue										208,873
Cost of goods sold
Total cost of goods sold										65,418
Gross profit										143,455
Operating costs
Sales and marketing										64,913
Total operating costs										148,551
Operating income										(5,096)
Income (loss) before taxes										(1,567)
Provision (benefit) for income taxes										(658)
Net income (loss)										$ (909)
Basic (in dollars per share)										$ (0.02)
Diluted (in dollars per share)										$ (0.02)
Product and license
Revenue
Total revenue									$ 184,173	$ 152,977	$ 147,257
Cost of goods sold
Total cost of goods sold									63,393	50,706	48,333
Product and license | Adjustments
Revenue
Total revenue										2,365
Cost of goods sold
Total cost of goods sold										605
Product and license | Balances without the adoption of Topic 606
Revenue
Total revenue										155,342
Cost of goods sold
Total cost of goods sold										51,311
Services and other
Revenue
Total revenue									70,397	59,303	46,034
Cost of goods sold
Total cost of goods sold									$ 18,569	14,107	$ 10,444
Services and other | Adjustments
Revenue
Total revenue										(5,772)
Services and other | Balances without the adoption of Topic 606
Revenue
Total revenue										53,531
Cost of goods sold
Total cost of goods sold										14,107
As Previously Reported
Current assets
Accounts receivable, net of allowance					59,631					59,631
Contract asset					7,962					7,962
Other current assets					5,705					5,705
Total current assets					191,956					191,956
Deferred income taxes					5,601					5,601
Contract assets - non-current					3,316					3,316
Other assets					8,400					8,400
Total assets					352,826					352,826
Current liabilities
Deferred revenue					33,633					33,633
Short-term income taxes payable					6,905					6,905
Other accrued expenses					9,323					9,323
Total current liabilities					72,357					72,357
Deferred revenue - non-current					10,672					10,672
Deferred income taxes					2,661					2,661
Total liabilities					100,385					100,385
Stockholders' equity
Accumulated income					172,378					172,378
Total stockholders' equity					252,441					252,441
Total liabilities and stockholders' equity					$ 352,826					352,826
Revenue
Total revenue										212,280
Cost of goods sold
Total cost of goods sold										64,813
Gross profit										147,467
Operating costs
Sales and marketing										63,805
Total operating costs										147,443
Operating income										24
Income (loss) before taxes										3,553
Provision (benefit) for income taxes										(293)
Net income (loss)										$ 3,846
Basic (in dollars per share)										$ 0.10
Diluted (in dollars per share)										$ 0.10
As Previously Reported | Product and license
Revenue
Total revenue										$ 152,977
Cost of goods sold
Total cost of goods sold										50,706
As Previously Reported | Services and other
Revenue
Total revenue										59,303
Cost of goods sold
Total cost of goods sold										$ 14,107
ASU 2014-09
Stockholders' equity
Accumulated income												$ 11,900