Condensed Consolidated Statements Of Income - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Revenues:
Total revenues	$ 287,889	$ 273,174	$ 568,512	$ 527,843
Cost of revenues (1)	47,749	43,159	95,894	83,969
Gross profit	240,140	230,015	472,618	443,874
Operating expenses:
Sales and marketing (1)	83,171	80,862	169,482	158,339
Research, development and engineering (1)	11,252	11,555	23,462	23,307
General and administrative (1)	91,334	79,038	179,133	155,693
Total operating expenses	185,757	171,455	372,077	337,339
Income from operations	54,383	58,560	100,541	106,535
Interest (income) expense, net	15,502	13,670	31,254	26,079
Other expense, net	394	4,227	4,912	4,551
Income before income taxes and net loss in earnings of equity method investment	38,487	40,663	64,375	75,905
Income tax expense	7,037	9,287	14,055	18,709
Income (Loss) from Equity Method Investments	2,971	0	2,971	0
Net income	$ 28,479	$ 31,376	$ 47,349	$ 57,196
Net income per common share:
Basic	$ 0.59	$ 0.65	$ 0.98	$ 1.19
Diluted	$ 0.57	$ 0.63	$ 0.95	$ 1.16
Weighted average shares outstanding:
Basic	47,951,326	47,547,118	47,912,383	47,505,406
Diluted	49,218,521	48,948,315	48,962,835	48,857,405
Cash dividends paid per common share	$ 0.4150	$ 0.3750	$ 0.8200	$ 0.7400
Allocated Share-based Compensation Expense	$ 7,067	$ 5,562	$ 13,487	$ 9,177
Cost of Sales [Member]
Allocated Share-based Compensation Expense	129	121	250	238
Selling and Marketing Expense [Member]
Allocated Share-based Compensation Expense	467	521	832	899
Research and Development Expense [Member]
Allocated Share-based Compensation Expense	355	281	788	518
General and Administrative Expense [Member]
Allocated Share-based Compensation Expense	$ 6,116	$ 4,639	$ 11,617	$ 7,522