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Fair Value Measurements (Carrying amounts and fair value of financial instruments) (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Fair Value, Balance Sheet Grouping, Financial Statement Captions    
Unsecured revolving credit facilities - Carrying Amount $ 0 $ 115,630
Deferred financing costs, net and other 74,352 100,640
Senior Unsecured Notes    
Fair Value, Balance Sheet Grouping, Financial Statement Captions    
Unsecured debt, gross 850,000 850,000
Deferred financing costs, net and other 6,386 4,423
Unsecured Debt    
Fair Value, Balance Sheet Grouping, Financial Statement Captions    
Unsecured debt, gross 750,000 375,000
Deferred financing costs, net and other 1,266 2,785
Carrying Amount    
Fair Value, Balance Sheet Grouping, Financial Statement Captions    
Cash equivalents - Carrying Amount 1,500,227 1,307,105
Mortgages payable, gross 8,203,839 8,206,680
Unsecured revolving credit facilities - Carrying Amount 0 115,630
Long-term debt - Carrying Amount 9,803,839 9,547,310
Carrying Amount | Senior Unsecured Notes    
Fair Value, Balance Sheet Grouping, Financial Statement Captions    
Unsecured debt, gross 850,000 850,000
Carrying Amount | Unsecured Debt    
Fair Value, Balance Sheet Grouping, Financial Statement Captions    
Unsecured debt, gross 750,000 375,000
Fair Value    
Fair Value, Balance Sheet Grouping, Financial Statement Captions    
Long-term debt - Fair Value 9,822,000 9,553,000
Fair Value | Level 1    
Fair Value, Balance Sheet Grouping, Financial Statement Captions    
Cash Equivalents - Fair Value 1,500,000 1,307,000
Unsecured revolving credit facilities - Fair Value 0 116,000
Fair Value | Level 2    
Fair Value, Balance Sheet Grouping, Financial Statement Captions    
Mortgages payable, gross 8,194,000 8,163,000
Fair Value | Senior Unsecured Notes | Level 2    
Fair Value, Balance Sheet Grouping, Financial Statement Captions    
Unsecured debt, gross 878,000 899,000
Fair Value | Unsecured Debt | Level 1    
Fair Value, Balance Sheet Grouping, Financial Statement Captions    
Unsecured debt, gross $ 750,000 $ 375,000