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Leasing Activity (Narrative) (Details) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Jun. 30, 2019
Jun. 30, 2018
Operating Leased Assets [Line Items]        
Rental income, variable lease payments $ 24.0 $ 23.0 $ 49.0 $ 46.2
Percentage of leased asset subjected to termination options 3.30%   3.30%  
Percentage of leased asset exercisable in period 1.40%   1.40%  
Maximum [Member]        
Operating Leased Assets [Line Items]        
Non-cancelable lease term 10 years   10 years  
Minimum [Member]        
Operating Leased Assets [Line Items]        
Non-cancelable lease term 1 year   1 year