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Properties Held for Sale and Property Dispositions (Property Dispositions) (Details)
$ in Millions
1 Months Ended 6 Months Ended
Apr. 30, 2017
USD ($)
ft²
Feb. 29, 2016
USD ($)
ft²
Jun. 30, 2017
USD ($)
ft²
Properties Sold [Abstract]      
Approximate Square Feet (sqft/acres) | ft²     32,134,288
885 Third Avenue      
Properties Sold [Abstract]      
Approximate Square Feet (sqft/acres) | ft²   607,000  
Sales Price   $ 453.0  
Gain on Sale (Loss)   $ (8.8)  
520 White Plains Road      
Properties Sold [Abstract]      
Approximate Square Feet (sqft/acres) | ft² 180,000    
Sales Price $ 21.0    
Gain on Sale (Loss) $ (14.6)    
102 Greene Street      
Properties Sold [Abstract]      
Approximate Square Feet (sqft/acres) | ft² 9,000    
Sales Price $ 43.5    
Gain on Sale (Loss) $ 4.9    
Employee compensation award     $ 0.9
Joint venture, ownership percentage sold 90.00%