Basis and Summary of Significant Accounting Policies - Statements of Stockholders' Equity (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018	Jan. 01, 2019
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	$ 1,813,508	$ 1,760,708	$ 1,756,192	$ 1,728,004	$ 1,760,708	$ 1,728,004
Dividends	(2,386)	(2,363)	(2,087)	(2,146)
Stock-Based Compensation	6,522	7,050	6,380	7,862
Reissued Shares	3,060	(10,981)	(3,613)	(9,161)
Repurchased Shares	(14,414)		(50,025)	(18,407)
Net Earnings	42,650	56,078	38,501	52,246	98,728	90,747
Foreign Currency Translation Adjustment	618	424	(535)	(477)	1,042	(1,012)
Ending Balance	1,849,558	1,813,508	1,744,813	1,756,192	1,849,558	1,744,813
Treasury Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	$ (563,583)	$ (567,847)	$ (422,679)	$ (407,713)	$ (567,847)	$ (407,713)
Beginning Balance (in shares)	(23,075)	(23,568)	(20,579)	(20,733)	(23,568)	(20,733)
Reissued Shares (in shares)	113	493	220	545
Reissued Shares	$ 2,776	$ 4,264	$ 1,795	$ 3,441
Repurchased Shares (in shares)	(243)		(1,234)	(391)
Repurchased Shares	$ (14,414)		$ (50,025)	$ (18,407)
Ending Balance (in shares)	(23,205)	(23,075)	(21,593)	(20,579)	(23,205)	(21,593)
Ending Balance	$ (575,221)	$ (563,583)	$ (470,909)	$ (422,679)	$ (575,221)	$ (470,909)
Common Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	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
Additional Paid-in Capital
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	270,727	278,922	265,303	270,043	278,922	270,043
Stock-Based Compensation	6,522	7,050	6,380	7,862
Reissued Shares	284	(15,245)	(5,408)	(12,602)
Ending Balance	$ 277,533	$ 270,727	$ 266,275	$ 265,303	277,533	266,275
Retained Earnings
Dividends, per share (in dollars per share)	$ 0.035	$ 0.03	$ 0.03	$ 0.035
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	$ 2,061,651	$ 2,005,344	$ 1,867,895	$ 1,819,524	2,005,344	1,819,524
Dividends	(2,386)	(2,363)	(2,087)	(2,146)
Net Earnings	42,650	56,078	38,501	52,246
Ending Balance	2,101,915	2,061,651	1,904,309	1,867,895	2,101,915	1,904,309
Accumulated Other Comprehensive Income (Loss)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	(663)	(1,087)	297	774	(1,087)	774
Foreign Currency Translation Adjustment	618	424	(535)	(477)
Ending Balance	$ (45)	(663)	$ (238)	297	$ (45)	$ (238)
Accounting Standards Update 2014-09
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Opening Balance Sheet Adjustment - ASC 842, net of taxes				(1,729)
Accounting Standards Update 2014-09 | Retained Earnings
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Opening Balance Sheet Adjustment - ASC 842, net of taxes				$ (1,729)
Accounting Standards Update 2016-02
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Opening Balance Sheet Adjustment - ASC 842, net of taxes		2,592
Accounting Standards Update 2016-02 | Retained Earnings
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Opening Balance Sheet Adjustment - ASC 842, net of taxes		$ 2,592					$ 2,600