Earnings Per Share, Textuals (Details) (USD $)	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Potential dilutive effect of options and Warrants
Options outstanding to purchase common shares	16,645		16,645
Average exercise price per share	$ 80.01		$ 80.01
Trading days warrants are exercisable	120 days
Add: dilutive effect of warrants	0	471,000
Dividends
Cash dividends paid	$ 7,100,000us-gaap_PaymentsOfDividendsCommonStock	$ 6,400,000us-gaap_PaymentsOfDividendsCommonStock
Cash dividends paid per share	$ 0.40	$ 0.35
Treasury Stock, Number of Shares and Restriction Disclosures [Abstract]
Repurchase of common stock	405,259us-gaap_TreasuryStockSharesAcquired	184,400us-gaap_TreasuryStockSharesAcquired
Weighted-average price per share of common stock repurchased	$ 74.05	$ 69.07
Repurchase of common stock, total cost	30,000,000	12,700,000
Common share amount available for additional share repurchase	$ 42,700,000		$ 72,800,000
Warrant Transactions
Potential dilutive effect of options and Warrants
Number of common shares underlying the Warrants outstanding	3,700,000
Average exercise price of common shares underlying Warrants	$ 60.57
Warrant [Member]
Potential dilutive effect of options and Warrants
Number of common shares underlying the Warrants outstanding		3,600,000	3,600,000
Average exercise price of common shares underlying Warrants		$ 60.98
Warrants [Member]
Potential dilutive effect of options and Warrants
Add: dilutive effect of warrants		470,735

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Exercise price per share or per unit of warrants or rights outstanding.

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Aggregate dividends paid during the period for each share of common stock outstanding.

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Additional shares included in the calculation of diluted EPS as a result of the potentially dilutive effect of call options and warrants using the treasury stock method.

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Amount of cash outflow in the form of ordinary dividends to common shareholders of the parent entity.

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Weighted average price at which grantees can acquire the shares reserved for issuance under the stock option plan.

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As of the balance sheet date, the number of shares into which fully vested and expected to vest stock options outstanding can be converted under the option plan.

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Amount remaining of a stock repurchase plan authorized.

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Total cost of shares repurchased divided by the total number of shares repurchased.

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Equity impact of the cost of common and preferred stock that were repurchased during the period. Recorded using the cost method.

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