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, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Net Income (Loss) Available to Common Stockholders, Basic [Abstract]
Net income (numerator for basic and diluted calculations)	$ 130,391	$ 198,296	$ 255,505	$ 334,960
Shares:
Weighted average outstanding shares (denominator for basic calculation)	66,351	69,076	67,246	69,823
Equivalent shares from outstanding stock options	769	642	785	666
Denominator for diluted calculation	67,120	69,718	68,031	70,489
Earnings Per Share:
Basic earnings per share	$ 1.97	$ 2.87	$ 3.80	$ 4.80
Diluted earnings per share	$ 1.94	$ 2.84	$ 3.76	$ 4.75