Quarterly Financial Data (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Employee Service Share-based Compensation, Tax Benefit Realized from Exercise of Stock Options									$ 7,556	$ 9,330	$ 5,700
Weighted Average Number Diluted Shares Outstanding Adjustment									1,086,000	648,000	365,000
Revenue, Net	$ 95,688	$ 97,108	$ 96,721	$ 89,855	$ 90,387	$ 86,016	$ 83,781	$ 81,484
Gross Profit	50,760	51,273	50,132	$ 49,233	48,257	46,265	43,761	42,514	$ 201,398	$ 180,797	$ 151,401
Net Income	$ 9,512	$ 18,806	$ 16,606		$ 5,463	$ 16,266	$ 13,570	$ 9,686	$ 63,084	$ 44,985	$ 26,335
Basic	$ 0.58	$ 1.16	$ 1.03	$ 0.99	$ 0.34	$ 1.02	$ 0.86	$ 0.62	$ 3.90	$ 2.84	$ 1.72
Diluted	$ 0.54	$ 1.09	$ 0.98	$ 0.96	$ 0.33	$ 0.98	$ 0.83	$ 0.60	$ 3.66	$ 2.73	$ 1.68
ASU 2016-09 [Member]
Weighted Average Number Diluted Shares Outstanding Adjustment									413,000
Adjustments for New Accounting Pronouncement [Member]
Employee Service Share-based Compensation, Tax Benefit Realized from Exercise of Stock Options				$ 2,300
Weighted Average Number Diluted Shares Outstanding Adjustment				314,000
Net Income				$ 18,160
Basic				$ 1.13
Diluted				$ 1.08