Earnings Per Share (Basic and Diluted Earnings) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended	9 Months Ended
Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Earnings Per Share [Abstract]
Net income	$ 130,801	$ 83,379	$ 324,140	$ 247,816
Weighted average number of common shares outstanding	31,020	30,897	31,043	30,937
Dilutive stock awards	527	399	459	297
Diluted weighted average number of common shares outstanding	31,547	31,296	31,502	31,234
Basic earnings per share	$ 4.22	$ 2.70	$ 10.44	$ 8.01
Diluted earnings per share	$ 4.15	$ 2.66	$ 10.29	$ 7.93

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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