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Fair Value Measurements (Details) - USD ($)
$ / shares in Units, $ in Millions
3 Months Ended 6 Months Ended 12 Months Ended
Mar. 31, 2017
Jun. 30, 2018
Dec. 31, 1991
Dec. 31, 2017
Apr. 18, 2017
Public Preferred Stock [Member]          
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Abstract]          
Public preferred stock par value (in dollar per share)   $ 0.01      
Preferred stock dividend rate per annum   12.00% 6.00%    
Senior Redeemable Preferred Stock [Member]          
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Abstract]          
Redemption amount of senior redeemable preferred stock         $ 2.1
Preferred stock dividend rate per annum 14.125%        
Carrying (Reported) Amount, Fair Value Disclosure [Member] | Public Preferred Stock [Member]          
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Abstract]          
Public preferred stock   $ 133.5   $ 131.6  
Estimate of Fair Value, Fair Value Disclosure [Member] | Public Preferred Stock [Member]          
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Abstract]          
Public preferred stock   $ 42.4   $ 42.2