Fair Value Measurements (Details 2) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Fair Value Disclosures [Abstract]
Carrying Value, Mortgage and notes payable	$ 5,274,583	$ 5,894,443
Carrying Value, Margins payable	22,097	139,142
Estimated Fair Value, Mortgage and notes payable	5,317,999	5,790,201
Estimated Fair Value, Margins payable	$ 22,097	$ 139,142