Noncontrolling Interests - Variable interest entity (Details) - Variable Interest Entities, Primary Beneficiary, Aggregated Disclosure - ASU 2015-02 - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Variable interest entity
Net real estate	$ 2,936,397	$ 3,040,903
Total assets	3,342,443	3,499,481
Total debt	210,384	251,047
Total liabilities	$ 313,295	$ 364,797