Earnings Per Share (Reconciliation of the Numerators and Denominators of the Basic and Diluted Per Shares) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Earnings:
Net income (numerator for basic and diluted calculations)	$ 136,664	$ 185,535
Shares:
Weighted average outstanding shares (denominator for basic calculation)	70,574	73,838
Equivalent shares from outstanding stock options	690	551
Denominator for diluted calculation	71,264	74,389
Earnings Per Share:
Basic earnings per share	$ 1.94	$ 2.51
Diluted earnings per share	$ 1.92	$ 2.49