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Real Estate (Impairments) (Details)
$ in Millions
12 Months Ended
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
property
Property, Plant and Equipment [Line Items]      
Impairment of real estate | $ $ 11.9 $ 10.7 $ 5.9
Commercial Operating Properties      
Property, Plant and Equipment [Line Items]      
Number of properties impaired     2
Residential Operating Properties      
Property, Plant and Equipment [Line Items]      
Number of properties impaired     1