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Summary of Significant Accounting Policies (Stock-Based Compensation) (Narrative) (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Restricted stock awards        
Stock-based compensation        
Total unrecognized compensation cost $ 6.6   $ 6.6  
Remaining weighted average vesting period for recognition of unrecognized compensation cost     1 year 10 months 8 days  
Recognized compensation expense 1.1 $ 1.2 $ 3.5 $ 4.7
Performance-based restricted stock awards        
Stock-based compensation        
Total unrecognized compensation cost 11.0   $ 11.0  
Remaining weighted average vesting period for recognition of unrecognized compensation cost     1 year 9 months 28 days  
Recognized compensation expense $ 2.1 $ 2.4 $ 4.3 $ 7.2
Period of total shareholder return upon which the percentage of shares vesting at the end of the measurement period will be based     3 years  
Period of return of the MSCI US REIT index against which total shareholder return measured     3 years  
Percentage of revenues from triple-net leases (at least)     75.00%  
End Of Measurement Period Vesting | Performance-based restricted stock awards        
Stock-based compensation        
Vesting period     3 years