Amount due from customers or clients, within one year of the balance sheet date (or the normal operating cycle, whichever is longer), for goods or services (including trade receivables) that have been delivered or sold in the normal course of business, reduced to the estimated net realizable fair value by an allowance established by the entity of the amount it deems uncertain of collection.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 8
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=109196051&loc=SL49130549-203045

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(3))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Basis of presentation - Effect of Adoption on Income Statement and Balance Sheet (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Jan. 01, 2018	Dec. 31, 2017
Consolidated Statements of Income
Total net sales	$ 341,009	$ 318,609	$ 652,906	$ 618,714
Operating Expenses	(221,938)	(207,518)	(431,176)	(408,782)
Operating Income	36,912	28,631	65,048	50,948
Provision for income taxes	3,948	4,197	9,292	9,223
Net income	$ 31,054	$ 25,155	$ 55,322	$ 43,303
Basic earnings per share (in dollars per share)	$ 0.24	$ 0.19	$ 0.42	$ 0.33
Diluted earnings per share (in dollars per share)	$ 0.23	$ 0.19	$ 0.42	$ 0.33
Assets [Abstract]
Accounts receivable, net	$ 249,910		$ 249,910		$ 251,224	$ 248,825
Prepaid expenses and other current assets	56,740		56,740			48,621
Deferred revenue - current	125,451		125,451		111,571	120,638
Deferred revenue - non-current	31,550		31,550		32,745	33,742
Other current liabilities	14,795		14,795		25,882	23,782
Deferred income taxes	37,381		37,381		35,247	33,609
Retained earnings	316,607		316,607		321,860	$ 313,241
Accounting Standards Update 2014-09 \| ASC 606 Adjustments
Consolidated Statements of Income
Total net sales	1,042		7,454
Operating Expenses	(183)		(206)
Operating Income	1,225		7,660
Provision for income taxes	124		1,282
Net income	$ 1,101		$ 6,378
Basic earnings per share (in dollars per share)	$ 0.01		$ 0.05
Diluted earnings per share (in dollars per share)	$ 0.01		$ 0.05
Assets [Abstract]
Accounts receivable, net	$ 2,093		$ 2,093		2,399
Prepaid expenses and other current assets	100		100
Deferred revenue - current	(14,276)		(14,276)		(9,067)
Deferred revenue - non-current	(3,491)		(3,491)		(997)
Other current liabilities	3,382		3,382		2,100
Deferred income taxes	1,638		1,638		1,638
Retained earnings	14,940		14,940		$ 8,619
Product
Consolidated Statements of Income
Total net sales	306,780	$ 289,817	587,139	$ 561,328
Product \| Accounting Standards Update 2014-09 \| ASC 606 Adjustments
Consolidated Statements of Income
Total net sales	1,042		7,454
Software maintenance
Consolidated Statements of Income
Total net sales	34,229	$ 28,792	65,767	$ 57,386
Software maintenance \| Accounting Standards Update 2014-09 \| ASC 606 Adjustments
Consolidated Statements of Income
Total net sales	$ 0		$ 0