Earnings Per Share (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014
Earnings Per Share, Basic and Diluted
Net earnings	$ 112,917us-gaap_NetIncomeLoss	$ 107,911us-gaap_NetIncomeLoss
Less: net earnings attributable to unvested restricted stock	1,641	1,692
Net earnings available to common stock	111,276	106,219
Weighted-average common shares outstanding	94,881	98,662
Dilutive effect of outstanding securities (determined using treasury stock method)	316	251
Weighted-average common shares and potential common shares outstanding	95,197	98,913
Earnings Per Share, Basic	$ 1.17us-gaap_EarningsPerShareBasic	$ 1.08us-gaap_EarningsPerShareBasic
Earnings Per Share, Diluted	$ 1.17us-gaap_EarningsPerShareDiluted	$ 1.07us-gaap_EarningsPerShareDiluted
Net earnings available to common stock	$ 111,276	$ 106,219

The amount of net income (loss) for the period per each share of common stock or unit outstanding during the reporting period.

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The amount of net income (loss) for the period available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period.

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Additional shares included in the calculation of diluted EPS as a result of the potentially dilutive effect of share based payment arrangements using the treasury stock method.

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The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

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Net income after adjustments for dividends on preferred stock (declared in the period) and/or cumulative preferred stock (accumulated for the period).

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Net Income or Loss Available to Common Stockholders plus adjustments resulting from the assumption that dilutive convertible securities were converted, options or warrants were exercised, or that other shares were issued upon the satisfaction of certain conditions.

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Amount of undistributed earnings (loss) allocated to participating securities for the basic earnings (loss) per share or per unit calculation under the two-class method.

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