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Equity - Incentive and Share-Based Compensation (Details)
$ / shares in Units, $ in Millions
1 Months Ended 3 Months Ended 9 Months Ended 12 Months Ended
May 31, 2018
USD ($)
shares
Mar. 31, 2018
USD ($)
vesting_installment
shares
Feb. 28, 2018
USD ($)
vesting_installment
shares
Sep. 30, 2018
USD ($)
shares
Sep. 30, 2017
USD ($)
Sep. 30, 2018
USD ($)
$ / shares
shares
Sep. 30, 2017
USD ($)
Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Expected volatility rate look-back period           6 years    
Weighted-average per unit or share fair value (in USD per share) | $ / shares           $ 8.54    
Age of grantee at which LTIP unit and restricted stock awards immediately vest           60 years    
Period of service, upon completion of which, grantee's LTIP unit and restricted stock awards will immediately vest           10 years    
2013 Plan                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Number of shares authorized under the plan (in shares) | shares       12,200,000.0   12,200,000.0    
Number of shares that remain available for future issuance (in shares) | shares       4,300,000   4,300,000    
Long-Term Incentive Plan Unit and Restricted Stock                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Fair value of share-based awards granted in period | $           $ 23.6    
Weighted-average per unit or share fair value (in USD per share) | $ / shares           $ 8.54    
Expected term           5 years 6 months 24 days    
Dividend rate (as a percent)           2.30%    
Risk-free interest rate (as a percent)           2.50%    
Expected price volatility (as a percent)           17.00%    
2013 Plan Units, granted in March 2018 | Time Based Long-Tern Incentive Plan Unit                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Granted (in shares) | shares   67,449            
Fair value of share-based awards granted in period | $   $ 1.7            
Award vesting period   4 years            
Number of vesting installments | vesting_installment   3            
2013 Plan Units, granted in March 2018 | Performance Based Long-Term Incentive Plan Unit                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Granted (in shares) | shares   223,950            
Fair value of share-based awards granted in period | $   $ 1.1            
Award vesting period   3 years            
Number of vesting installments | vesting_installment   2            
2013 Plan Units, granted in March 2018 | Time Restricted Shares                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Granted (in shares) | shares   39,608            
Awards that Meet Age and Service Requirements for Vesting                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Noncash share-based compensation expense recognized | $       $ 0.4 $ 0.2 $ 1.7 $ 0.9  
Unrecognized compensation expense | $       1.2   $ 1.2    
Unrecognized compensation expense, period for recognition           2 years 2 months    
Awards that Do Not Meet Age and Service Requirements for Vesting                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Noncash share-based compensation expense recognized | $       4.5 $ 3.7 $ 12.4 $ 9.9  
Unrecognized compensation expense | $       $ 32.0   $ 32.0    
Unrecognized compensation expense, period for recognition           2 years 120 days    
Executive Officer | 2013 Plan Units, granted in March 2018 | Time Based Long-Tern Incentive Plan Unit                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Granted (in shares) | shares   386,876            
Fair value of share-based awards granted in period | $   $ 6.1            
Executive Officer | 2013 Plan Units, granted in March 2018 | Performance Based Long-Term Incentive Plan Unit                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Granted (in shares) | shares   1,737,917            
Fair value of share-based awards granted in period | $   $ 9.6            
Executive Officer | 2013 Plan Units, granted in February 2018 | Time Based Long-Tern Incentive Plan Unit                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Granted (in shares) | shares     238,609          
Fair value of share-based awards granted in period | $     $ 4.0          
Award vesting period               3 years
Vesting percentage of award face amount               125.00%
Executive Officer | Vest immediately | Time Based Long-Tern Incentive Plan Unit                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Granted (in shares) | shares     25,158          
Executive Officer | Vest ratably over three years | Time Based Long-Tern Incentive Plan Unit                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Granted (in shares) | shares     213,451          
Award vesting period     3 years          
Executive Officer | Vest in two equal annual installments | Time Based Long-Tern Incentive Plan Unit                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Number of vesting installments | vesting_installment     2          
Non-employee Directors | 2013 Plan Units, granted in February 2018 | Time Based Long-Tern Incentive Plan Unit                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Granted (in shares) | shares 65,000              
Fair value of share-based awards granted in period | $ $ 1.0              
Non-employee Directors | Vest ratably over three years | Time Based Long-Tern Incentive Plan Unit                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Award vesting period 3 years