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Issuance and Redemption of Preferred Stock (Details) (USD $)
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12 Months Ended | 1 Months Ended | 12 Months Ended | 1 Months Ended | 12 Months Ended | 1 Months Ended | 12 Months Ended | |||||||||||
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Dec. 31, 2012
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Dec. 31, 2011
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Dec. 31, 2010
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Mar. 31, 2012
Class D Cumulative Redeemable Preferred Stock
dividend
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Dec. 31, 2012
Class D Cumulative Redeemable Preferred Stock
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Dec. 31, 2011
Class D Cumulative Redeemable Preferred Stock
dividend
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Dec. 31, 2010
Class D Cumulative Redeemable Preferred Stock
dividend
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Dec. 31, 2004
Class D Cumulative Redeemable Preferred Stock
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Jan. 31, 2013
Class E Cumulative Redeemable Preferred Stock
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Dec. 31, 2012
Class E Cumulative Redeemable Preferred Stock
dividend
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Dec. 31, 2011
Class E Cumulative Redeemable Preferred Stock
dividend
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Dec. 31, 2010
Class E Cumulative Redeemable Preferred Stock
dividend
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Dec. 31, 2006
Class E Cumulative Redeemable Preferred Stock
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Feb. 29, 2012
6.625% Class D Cumulative Redeemable Preferred Stock
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Jan. 31, 2013
Class F Cumulative Redeemable Preferred Stock
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Apr. 30, 2012
Class F Cumulative Redeemable Preferred Stock
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Mar. 31, 2012
Class F Cumulative Redeemable Preferred Stock
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Dec. 31, 2012
Class F Cumulative Redeemable Preferred Stock
dividend
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| Issuance and redemption of preferred stock | ||||||||||||||||||
| Shares issued in stock offerings, net of offering costs (in shares) | 5,100,000 | 8,800,000 | 14,950,000 | 1,400,000 | ||||||||||||||
| Number of preferred stock shares redeemed | 5,100,000 | |||||||||||||||||
| Preferred stock, dividend rate (as a percent) | 7.375% | 7.375% | 6.75% | 6.625% | ||||||||||||||
| Redemption amount (in dollars per share) | $ 25.00 | $ 25.00 | $ 25.00 | |||||||||||||||
| Original issuance costs | $ 3,700,000 | |||||||||||||||||
| Number of monthly dividends paid | 2 | 12 | 12 | 12 | 12 | 12 | 10 | |||||||||||
| Dividend paid per preferred share of stock (in dollars per share) | $ 0.3841147 | $ 1.8437508 | $ 0.140625 | $ 1.6875 | $ 1.6875 | $ 1.6875 | $ 0.138021 | $ 1.4124147 | ||||||||||
| Dividend paid per preferred share of stock in arrears (in dollars per share) | $ 0.138021 | |||||||||||||||||
| Preferred stock, dividends paid | 2,000,000 | 2,000,000 | 9,400,000 | 9,400,000 | 14,900,000 | 14,900,000 | 14,900,000 | 22,600,000 | ||||||||||
| Offering price per share (in dollars per share) | $ 25 | $ 25.2863 | ||||||||||||||||
| Preferred stock, purchased by underwriters (in shares) | 1,950,000 | |||||||||||||||||
| Underwriting discounts and other offering costs | 13,773,000 | 25,200,000 | 22,471,000 | 13,800,000 | ||||||||||||||
| Net proceeds from preferred stock offering | 395,377,000 | 395,400,000 | ||||||||||||||||
| Redemption of preferred stock | $ 127,500,000 | $ 127,500,000 | ||||||||||||||||
| Dividend paid per initial preferred share of stock (in dollars per share) | $ 0.1702257 | |||||||||||||||||
| Period when initial dividend payments were made | 37 days | |||||||||||||||||