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Shareholders' Equity (Details) (USD $) In Thousands, except Share data, unless otherwise specified | 3 Months Ended | 6 Months Ended |  | 0 Months Ended
--- | --- | --- | --- | ---
Jun. 30, 2012 | Jun. 30, 2011 | Jun. 30, 2012 | Jun. 30, 2011 | Dec. 31, 2011 | 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
Net proceeds from issuance of Series L preferred shares |  |  | $ 165,902 | $ 0 |  | $ 165,700 |
Series L cumulative preferred shares, redemption price per share |  |  |  |  |  | $ 25.00 |
Preferred stock, liquidation preference value in aggregate | $ 388,833 |  | $ 388,833 |  | $ 216,333 | $ 172,500 |
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) |  |  | 54,550 |  |  |  |
Common Stock, Conversion Basis for Common Unit |  |  | 1 |  |  |  |
Dividends declared per common share (in dollars per share) | $ 0.275 | $ 0.4125 | $ 0.55 | $ 0.825 |  |  |

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