Fair Value Measurements (Details 2) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior unsecured notes	$ 847,463	$ 1,347,159
Carrying (Reported) Amount, Fair Value Disclosure [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash equivalents	311,017	749,418
Mortgage and mezzanine loans receivable	0	16,748
Total assets	311,017	766,166
Mortgages payable	8,562,314	8,263,165
Senior unsecured notes	847,463	1,347,159
Revolving credit facility debt	400,000	0
Total debt	9,809,777	9,610,324
Estimate of Fair Value, Fair Value Disclosure [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash equivalents	311,000	749,000
Mortgage and mezzanine loans receivable	0	17,000
Total assets	311,000	766,000
Mortgages payable	8,541,000	8,224,000
Senior unsecured notes	882,000	1,385,000
Revolving credit facility debt	400,000	0
Total debt	$ 9,823,000	$ 9,609,000