Basis of Presentation and Summary of Significant Accounting Policies - Investment Properties (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Investment properties, at cost:
Land, buildings and improvements	$ 3,600,743	$ 3,904,291
Furniture, equipment and other	7,741	8,453
Construction in progress	32,636	45,140
Investment properties, at cost	$ 3,641,120	$ 3,957,884