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Fair Value Measurements (Carrying amounts and fair value of financial instruments) (Details) - USD ($)
$ in Thousands
Dec. 31, 2016
Dec. 31, 2015
Fair Value, Balance Sheet Grouping, Financial Statement Captions    
Unsecured revolving credit facilities - Carrying Amount $ 115,630 $ 550,000
Senior Unsecured Notes    
Fair Value, Balance Sheet Grouping, Financial Statement Captions    
Unsecured debt, gross 850,000 850,000
Unsecured Debt    
Fair Value, Balance Sheet Grouping, Financial Statement Captions    
Unsecured debt, gross 375,000 187,500
Carrying Amount    
Fair Value, Balance Sheet Grouping, Financial Statement Captions    
Cash equivalents - Carrying Amount 1,307,105 1,295,980
Mortgages payable, gross 9,374,297 9,614,838
Unsecured revolving credit facilities - Carrying Amount 115,630 550,000
Debt instrument - Carrying amount 10,714,927 11,202,338
Carrying Amount | Senior Unsecured Notes    
Fair Value, Balance Sheet Grouping, Financial Statement Captions    
Loans Receivable - Carrying Amount 850,000 850,000
Carrying Amount | Unsecured Debt    
Fair Value, Balance Sheet Grouping, Financial Statement Captions    
Loans Receivable - Carrying Amount 375,000 187,500
Fair Value    
Fair Value, Balance Sheet Grouping, Financial Statement Captions    
Total debt 10,746,000 10,911,500
Fair Value | Level 1    
Fair Value, Balance Sheet Grouping, Financial Statement Captions    
Cash equivalents 1,307,000 1,296,000
Unsecured revolving credit facilities 116,000 550,000
Fair Value | Level 2    
Fair Value, Balance Sheet Grouping, Financial Statement Captions    
Mortgages payable, gross 9,356,000 9,306,000
Fair Value | Senior Unsecured Notes | Level 2    
Fair Value, Balance Sheet Grouping, Financial Statement Captions    
Unsecured debt, gross 899,000 868,000
Fair Value | Unsecured Debt | Level 1    
Fair Value, Balance Sheet Grouping, Financial Statement Captions    
Unsecured debt, gross $ 375,000 $ 187,500