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Shareholders' Equity (Details) (USD $)
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3 Months Ended | 0 Months Ended | 3 Months Ended | 1 Months Ended | 3 Months Ended | 1 Months Ended | 3 Months Ended | 3 Months Ended | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
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Mar. 31, 2013
period
director
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Mar. 31, 2012
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Dec. 31, 2012
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Mar. 15, 2013
Common Stock [Member]
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Feb. 20, 2013
Common Stock [Member]
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Mar. 15, 2012
Common Stock [Member]
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Mar. 31, 2013
Common Stock [Member]
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Dec. 31, 2006
Series D Preferred Stock [Member]
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Mar. 31, 2013
Series D Preferred Stock [Member]
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Dec. 27, 2012
Series D Preferred Stock [Member]
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Nov. 30, 2011
Series D Preferred Stock [Member]
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Mar. 31, 2004
Series C Preferred Stock [Member]
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Mar. 31, 2013
Series C Preferred Stock [Member]
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Mar. 23, 2009
Series C Preferred Stock [Member]
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Mar. 31, 2013
Retained Earnings [Member]
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Mar. 31, 2012
Retained Earnings [Member]
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| Class of Stock [Line Items] | ||||||||||||||||
| Dividends declared, Per common share (usd per share) | $ 0.28 | $ 0.27 | $ 0.28 | |||||||||||||
| Dividends paid, Per common share (usd per share) | $ 0.28 | |||||||||||||||
| Dividends declared and paid, Preference shares | $ 6,275,000 | $ 8,750,000 | $ 8,750,000 | |||||||||||||
| Dividends, Common Stock | 12,329,000 | 13,886,000 | ||||||||||||||
| Repurchase of shares | 1,400,000 | |||||||||||||||
| Common shares repurchased during period, Aggregate value | (111,300,000) | |||||||||||||||
| Common shares repurchased during period, Average cost per share | $ 81.29 | |||||||||||||||
| Share repurchase program, Authorized aggregate amount | 500,000,000 | |||||||||||||||
| Share repurchase program, Remaining authorized aggregate amount | 500,000,000 | |||||||||||||||
| Proceeds from issuance of preference shares | 300,000,000 | 250,000,000 | ||||||||||||||
| Preference shares issued, Shares | 12,000,000 | 10,000,000 | ||||||||||||||
| Equity Issuance, Per Share Amount | $ 25 | $ 25 | ||||||||||||||
| Preferred Stock, Redemption Price Per Share | $ 25 | $ 25 | ||||||||||||||
| Preference shares, Dividend rate | 6.60% | 6.08% | ||||||||||||||
| Preferred shares, Amount of preferred dividends in arrears, equivalent number of periods, trigger | 6 | |||||||||||||||
| Preferred shares, Preferred stock dividends in arrears, triggered shareholder election right, number of directors | 2 | |||||||||||||||
| Preferred shares, Redemption amount, shares | 6,000,000 | |||||||||||||||
| Preferred shares, Redemption mount | $ 150,000,000 | |||||||||||||||
| Preferred shares, Shares outstanding | 16,000,000 | 16,000,000 | 6,000,000 | |||||||||||||