Basis and Summary of Significant Accounting Policies - Statements of Stockholders' Equity (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018	Jan. 01, 2019	Jan. 01, 2018
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	$ 1,849,558	$ 1,813,508	$ 1,760,708	$ 1,744,813	$ 1,756,192	$ 1,728,004	$ 1,760,708	$ 1,728,004
Dividends	(2,363)	(2,386)	(2,363)	(2,068)	(2,087)	(2,146)
Stock-Based Compensation	5,911	6,522	7,050	6,140	6,380	7,862
Reissued Shares	59	3,060	(10,981)	2,176	(3,613)	(9,161)
Repurchased Shares	(25,008)	(14,414)		(31,572)	(50,025)	(18,407)
Net Earnings	39,801	42,650	56,078	43,720	38,501	52,246	138,529	134,467
Foreign Currency Translation Adjustment	(303)	618	424	297	(535)	(477)	739	(715)
Ending Balance	1,867,655	1,849,558	1,813,508	1,763,506	1,744,813	1,756,192	1,867,655	1,763,506
Treasury Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	$ (575,221)	$ (563,583)	$ (567,847)	$ (470,909)	$ (422,679)	$ (407,713)	$ (567,847)	$ (407,713)
Beginning Balance (in shares)	(23,205)	(23,075)	(23,568)	(21,593)	(20,579)	(20,733)	(23,568)	(20,733)
Reissued Shares (in shares)	2	113	493	104	220	545
Reissued Shares	$ 49	$ 2,776	$ 4,264	$ 2,322	$ 1,795	$ 3,441
Repurchased Shares (in shares)	(399)	(243)		(676)	(1,234)	(391)
Repurchased Shares	$ (25,008)	$ (14,414)		$ (31,572)	$ (50,025)	$ (18,407)
Ending Balance (in shares)	(23,602)	(23,205)	(23,075)	(22,165)	(21,593)	(20,579)	(23,602)	(22,165)
Ending Balance	$ (600,180)	$ (575,221)	$ (563,583)	$ (500,159)	$ (470,909)	$ (422,679)	$ (600,180)	$ (500,159)
Common Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	45,376	45,376	45,376	45,376	45,376	45,376	45,376	45,376
Ending Balance	45,376	45,376	45,376	45,376	45,376	45,376	45,376	45,376
Additional Paid-in Capital
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	277,533	270,727	278,922	266,275	265,303	270,043	278,922	270,043
Stock-Based Compensation	5,911	6,522	7,050	6,140	6,380	7,862
Reissued Shares	10	284	(15,245)	(146)	(5,408)	(12,602)
Ending Balance	$ 283,454	$ 277,533	$ 270,727	$ 272,269	$ 266,275	$ 265,303	283,454	272,269
Retained Earnings
Dividends, per share (in dollars per share)	$ 0.035	$ 0.035	$ 0.035	$ 0.03	$ 0.03	$ 0.03
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	$ 2,101,915	$ 2,061,651	$ 2,005,344	$ 1,904,309	$ 1,867,895	$ 1,819,524	2,005,344	1,819,524
Dividends	(2,363)	(2,386)	(2,363)	(2,068)	(2,087)	(2,146)
Net Earnings	39,801	42,650	56,078	43,720	38,501	52,246
Ending Balance	2,139,353	2,101,915	2,061,651	1,945,961	1,904,309	1,867,895	2,139,353	1,945,961
Accumulated Other Comprehensive Income (Loss)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	(45)	(663)	(1,087)	(238)	297	774	(1,087)	774
Foreign Currency Translation Adjustment	(303)	618	424	297	(535)	(477)
Ending Balance	$ (348)	$ (45)	$ (663)	$ 59	$ (238)	$ 297	$ (348)	$ 59
Accounting Standards Update 2014-09
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Opening Balance Sheet Adjustment - net of taxes										$ (1,729)
Accounting Standards Update 2014-09 | Retained Earnings
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Opening Balance Sheet Adjustment - net of taxes										$ (1,729)
Accounting Standards Update 2016-02
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Opening Balance Sheet Adjustment - net of taxes									$ 2,592
Accounting Standards Update 2016-02 | Retained Earnings
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Opening Balance Sheet Adjustment - net of taxes									$ 2,592