Fair Value Measurements (Details 2) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior unsecured notes	$ 1,791,814	$ 1,350,855
Carrying (Reported) Amount, Fair Value Disclosure [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash equivalents	1,157,000	295,000
Mortgage and mezzanine loans receivable	17,417	170,972
Total assets	1,174,417	465,972
Mortgages payable	8,988,843	8,331,993
Senior unsecured notes	1,791,814	1,350,855
Revolving credit facility debt	88,138	295,870
Total debt	10,868,795	9,978,718
Estimate of Fair Value, Fair Value Disclosure [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash equivalents	1,157,000	295,000
Mortgage and mezzanine loans receivable	17,000	171,000
Total assets	1,174,000	466,000
Mortgages payable	8,961,000	8,104,000
Senior unsecured notes	1,852,000	1,402,000
Revolving credit facility debt	88,000	296,000
Total debt	$ 10,901,000	$ 9,802,000