Annual report pursuant to Section 13 and 15(d)

Summary of Significant Accounting Policies - Income Taxes (Details)

v2.4.0.6
Summary of Significant Accounting Policies - Income Taxes (Details) (USD $)
In Millions, except Per Share data, unless otherwise specified
 12 Months Ended
Dec. 31, 2011
Dec. 31, 2010
Dec. 31, 2009
Schedule of Common Dividends Per Share [Line Items]      
REIT Annual Taxable Income Distribution Requirement Percentage 90.00%    
Parent Company [Member]
      
Schedule of Common Dividends Per Share [Line Items]      
Common Stock, Dividends, Per Share, Cash Paid $ 0.7938 $ 0.7725 $ 0.7638
Preferred Stock, Liquidation Preference Per Share   $ 25.00  
Preferred Stock, Dividends, Per Share, Cash Paid   $ 2.07 $ 1.88
Preferred Stock, Redemption Price Per Share   $ 25.00  
Preferred Stock, Redemption Price per Share Including All Accrued and Unpaid Dividends   $ 25.198  
Payments for Repurchase of Redeemable Preferred Stock, Including Accrued Dividends   $ 75.6  
Ordinary Income [Member] | Parent Company [Member]
      
Schedule of Common Dividends Per Share [Line Items]      
Preferred Stock, Dividends, Per Share, Cash Paid   $ 2.07 $ 1.85
Capital Gain [Member] | Parent Company [Member]
      
Schedule of Common Dividends Per Share [Line Items]      
Preferred Stock, Dividends, Per Share, Cash Paid     $ 0.03
Ordinary Income [Member] | Parent Company [Member]
      
Schedule of Common Dividends Per Share [Line Items]      
Common Stock, Dividends, Per Share, Cash Paid $ 0.7938 $ 0.5361 $ 0.7154
Capital Gain [Member] | Parent Company [Member]
      
Schedule of Common Dividends Per Share [Line Items]      
Common Stock, Dividends, Per Share, Cash Paid $ 0.0000 $ 0.0000 $ 0.0126
Return Of Captial [Member] | Parent Company [Member]
      
Schedule of Common Dividends Per Share [Line Items]      
Common Stock, Dividends, Per Share, Cash Paid $ 0.0000 $ 0.2364 $ 0.0358