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Shareholders' Equity (Details) (USD $)
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0 Months Ended | 3 Months Ended | 0 Months Ended | 1 Months Ended | 3 Months Ended | 1 Months Ended | 3 Months Ended | 0 Months Ended | ||||
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Mar. 31, 2012
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Feb. 22, 2012
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Mar. 31, 2012
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Mar. 31, 2011
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Mar. 15, 2012
Common Stock [Member]
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Dec. 31, 2006
Series D Preferred Stock [Member]
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Mar. 31, 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, 2012
Series C Preferred Stock [Member]
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Mar. 23, 2009
Series C Preferred Stock [Member]
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Nov. 17, 2010
Series B Preferred Stock [Member]
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| Class of Stock [Line Items] | ||||||||||||
| Dividends declared, Per common share | $ 0.27 | $ 0.26 | $ 0.27 | |||||||||
| Dividends paid, Per common share | $ 0.27 | |||||||||||
| Share repurchase program, Shares repurchased | 50,620 | |||||||||||
| Shares repurchased during period, Aggregate value | $ (3,600,000) | |||||||||||
| Shares repurchased during period, Average cost per share | $ 71.81 | |||||||||||
| 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 | ||||||||||
| Preference shares issued, Issuance price per share | $ 25 | $ 25 | ||||||||||
| Preference shares, Dividend rate | 6.60% | 6.08% | 7.30% | |||||||||
| Preference shares, Redemption price per share | $ 25 | $ 25 | $ 25 | |||||||||
| Dividends declared and paid, Preference shares | $ 8,750,000 | $ 8,750,000 | ||||||||||