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
	Mar. 31, 2018	Mar. 31, 2017
Net Income (Loss) Available to Common Stockholders, Basic [Abstract]
Net income (numerator for basic and diluted calculations)	$ 100,230	$ 145,512
Shares:
Weighted average outstanding shares (denominator for basic calculation)	64,490	64,353
Equivalent shares from outstanding stock options	1,382	1,318
Denominator for diluted calculation	65,872	65,671
Earnings Per Share:
Basic earnings per share	$ 1.55	$ 2.26
Diluted earnings per share	$ 1.52	$ 2.22