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Shareholders' Equity (Details) (USD $) | 9 Months Ended |  | 0 Months Ended | 3 Months Ended |  | 0 Months Ended
--- | --- | --- | --- | --- | --- | ---
Sep. 30, 2012 | Sep. 30, 2011 | Dec. 31, 2011 | Aug. 07, 2012 Series G Preferred Stock [Member] | Sep. 30, 2012 Series G Preferred Stock [Member] | Aug. 06, 2012 Series G Preferred Stock [Member] | Jun. 27, 2012 Public Offering Series L Cumulative Preferred Stock | Jun. 27, 2012 Public Offering Series L Preferred Units
Class of Stock [Line Items] |  |  |  |  |  |  |  |
Preferred shares issued to the public (in shares) | 6,900,000 | 4,600,000 |  |  |  |  | 6,900,000 |
Stock Issued During Period Per Share Value New Issues |  |  |  |  |  |  | $ 25.00 |
Preferred Stock |  |  |  |  |  |  |  |
Number of Series L preferred units of operating partnerships received in exchange for offering proceeds (in units) |  |  |  |  |  |  |  | 6,900,000
Preferred Stock, Redemption Price Per Share |  |  |  |  |  | $ 25.00 |  |
Stock Redeemed or Called During Period, Value | $ (55,000,000) |  |  | $ 55,000,000 |  |  |  |
Net proceeds from issuance of Series L preferred shares | 165,662,000 | 0 |  |  |  |  | 165,700,000 |
Series L and Series G cumulative preferred shares, redemption price per share |  |  |  |  |  |  | $ 25.00 |
Preferred stock, liquidation preference value in aggregate | 333,833,000 |  | 216,333,000 |  |  |  | 172,500,000 |
Issuance costs associated with redeemed preferred shares |  |  |  |  | $ 1,800,000 |  |  |
Annual dividend per share (in dollars per share) |  |  |  |  |  |  | $ 1.84375 |
Series L cumulative preferred shares, annual dividend rate (paid quarterly) |  |  |  |  |  |  | 7.375% |
Common Shares |  |  |  |  |  |  |  |
Number of common shares for each converted common unit (in shares) | 94,550 |  |  |  |  |  |  |
Common Stock, Conversion Basis for Common Unit | 1 |  |  |  |  |  |  |

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