Investments in real estate Land undergoing predevelopment and future development (Details) $ in Thousands	Dec. 31, 2015 USD ($) ft²	Dec. 31, 2014 USD ($)
Real Estate Properties [Line Items]
Land available for development | $	$ 283,761	$ 253,723
Square Footage of Real Estate Property, Future Development | ft²	11,965,247
Land held for future development in North America
Real Estate Properties [Line Items]
Land available for development | $	$ 283,800
Square Footage of Real Estate Property, Future Development | ft²	12,000,000