Operations and summary of significant accounting policies - Cumulative Effect Adjustment From Adoption of Topic 606 (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Jan. 01, 2018	Dec. 31, 2017
Assets [Abstract]
Accounts receivable, net	$ 248,872	$ 242,955	$ 251,224
Prepaid expenses and other current assets	65,477	54,337	33,618
Liabilities and stockholders' equity
Deferred revenue - current	131,445	127,288	111,571
Deferred revenue - non-current	33,480	32,636	32,745
Other current liabilities			25,882
Deferred income taxes	14,065	25,457	35,380
Retained earnings	$ 242,537	356,418	$ 322,898
Calculated under Revenue Guidance in Effect before Topic 606
Assets [Abstract]
Accounts receivable, net				$ 248,825
Prepaid expenses and other current assets				32,553
Liabilities and stockholders' equity
Deferred revenue - current				120,638
Deferred revenue - non-current				33,742
Other current liabilities				23,782
Deferred income taxes				33,609
Retained earnings				313,241
Accounting Standards Update 2014-09
Assets [Abstract]
Accounts receivable, net		2,093
Prepaid expenses and other current assets		1,220
Liabilities and stockholders' equity
Deferred revenue - current		(13,807)
Deferred revenue - non-current		(4,417)
Other current liabilities		3,399
Deferred income taxes		1,771
Retained earnings		$ 16,367
Accounting Standards Update 2014-09 | Difference between Revenue Guidance in Effect before and after Topic 606
Assets [Abstract]
Accounts receivable, net				2,399
Prepaid expenses and other current assets				1,065
Liabilities and stockholders' equity
Deferred revenue - current				(9,067)
Deferred revenue - non-current				(997)
Other current liabilities				2,100
Deferred income taxes				1,771
Retained earnings				$ 9,657