Fair Value Measurements (Details 2) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior unsecured notes	$ 847,594	$ 1,347,159
Carrying (Reported) Amount, Fair Value Disclosure [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash equivalents	545,617	749,418
Mortgage and mezzanine loans receivable (included in other assets)	0	16,748
Total assets	545,617	766,166
Mortgages payable	9,159,413	8,263,165
Senior unsecured notes	847,594	1,347,159
Revolving credit facility debt	0	0
Total debt	10,007,007	9,610,324
Estimate of Fair Value, Fair Value Disclosure [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash equivalents	546,000	749,000
Mortgage and mezzanine loans receivable (included in other assets)	0	17,000
Total assets	546,000	766,000
Mortgages payable	9,272,000	8,224,000
Senior unsecured notes	884,000	1,385,000
Revolving credit facility debt	0	0
Total debt	$ 10,156,000	$ 9,609,000