Earnings Per Share (Reconciliation of the Numerators and Denominators of the Basic and Diluted Per Shares) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Net Income (Loss) Available to Common Stockholders, Basic [Abstract]
Net income (numerator for basic and diluted calculations)	$ 202,698	$ 204,374	$ 372,205	$ 304,604
Shares:
Weighted average outstanding shares (denominator for basic calculation)	62,678	64,071	62,719	64,278
Equivalent shares from outstanding stock options	1,020	1,179	1,100	1,277
Denominator for diluted calculation	63,698	65,250	63,819	65,555
Earnings Per Share:
Basic earnings per share	$ 3.23	$ 3.19	$ 5.93	$ 4.74
Diluted earnings per share	$ 3.18	$ 3.13	$ 5.83	$ 4.65