Significant Accounting Policies (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Principles of Consolidation
Commercial real estate properties	$ 14,261,102	$ 12,163,976
Mortgages and other loans payable	6,865,383	5,586,709
Assets held for sale	117,885	462,430
Liabilities related to assets held for sale	94	266,873
Consolidated VIEs
Principles of Consolidation
Commercial real estate properties	198,600	198,400
Mortgages and other loans payable	$ 105,000	106,500
Commercial properties | Consolidated VIEs
Principles of Consolidation
Assets held for sale		445,000
Liabilities related to assets held for sale		$ 253,900