Operations and summary of significant accounting policies - Effect of Adoption on Income Statement and Balance Sheets (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Jan. 01, 2018
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
Income Statement [Abstract]
Total net sales	1,353,215	1,359,132	$ 1,289,386
Operating Expenses	(841,770)	(852,526)	(815,284)
Operating Income	174,554	172,879	145,778
Provision for income taxes	18,393	21,396	94,969
Net income	162,151	155,057	52,411
Product
Income Statement [Abstract]
Total net sales	$ 1,215,014	1,220,027	1,173,476
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
Accounting Standards Update 2014-09 | Product
Income Statement [Abstract]
Total net sales		7,911
Operating Expenses		(153)
Operating Income		8,064
Provision for income taxes		1,299
Net income		$ 6,765