Summary of Significant Accounting Policies - Investment Properties (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Investment properties
Capitalized interest	$ 24,754	$ 31,250	$ 32,664
Pre-development cost written off		$ 31,490
Buildings and improvements | Minimum
Investment properties
Useful life	10 years
Buildings and improvements | Maximum
Investment properties
Useful life	35 years
Equipment and fixtures | Minimum
Investment properties
Useful life	7 years
Equipment and fixtures | Maximum
Investment properties
Useful life	10 years