Earnings Per Share (Schedule of Earnings Per Share, Basic and Diluted) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Earnings Per Share, Basic and Diluted [Line Items]
Net income - numerator for earnings per common share	$ 169,328	$ 56,957	$ 404,193	$ 137,842
Weighted average common shares outstanding (shares)	90,485,567	89,068,770	90,278,209	88,872,562
Denominator for earnings per common share - assuming dilution (shares)	91,650,886	90,420,530	91,355,410	90,171,020
Earnings per common share	$ 1.87	$ 0.64	$ 4.48	$ 1.55
Earnings per common share - assuming dilution	$ 1.85	$ 0.63	$ 4.42	$ 1.53
Stock Options and Deferred Compensation Agreements
Earnings Per Share, Basic and Diluted [Line Items]
Effect of dilutive securities: share-based payment arrangements (shares)	705,465	974,208	750,588	924,465
Restricted Stock and Restricted Stock Units
Earnings Per Share, Basic and Diluted [Line Items]
Effect of dilutive securities: share-based payment arrangements (shares)	459,854	377,552	326,613	373,993
Stock Options
Earnings Per Share, Basic and Diluted [Line Items]
Antidilutive securities excluded from computation of earnings per share, amount (shares)	0	0	0	0

Securities (including those issuable pursuant to contingent stock agreements) that could potentially dilute basic earnings per share (EPS) or earnings per unit (EPU) in the future that were not included in the computation of diluted EPS or EPU because to do so would increase EPS or EPU amounts or decrease loss per share or unit amounts for the period presented.

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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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The average number of shares or units issued and outstanding that are used in calculating diluted EPS or earnings per unit (EPU), determined based on the timing of issuance of shares or units in the period.

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Number of [basic] shares or units, after adjustment for contingently issuable shares or units and other shares or units not deemed outstanding, determined by relating the portion of time within a reporting period that common shares or units have been outstanding to the total time in that period.

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