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Borrowings - Narrative (Details)
Sep. 09, 2019
USD ($)
Jul. 30, 2016
Jun. 30, 2020
USD ($)
Apr. 27, 2020
USD ($)
Apr. 26, 2020
USD ($)
Sep. 30, 2019
USD ($)
lender
Sep. 01, 2019
USD ($)
May 31, 2018
USD ($)
Jan. 31, 2010
USD ($)
Line of Credit Facility [Abstract]                  
Amount of available borrowings under the secured revolving credit facility     $ 250,000,000.0            
Senior Notes [Abstract]                  
Ownership interest In guarantor subsidiaries     100.00%            
Junior Subordinated Notes                  
Unsecured Debt [Abstract]                  
Stated interest rate on debt instrument (percent)   7.987%              
Junior Subordinated Notes [Abstract]                  
Effective period of debt instrument interest rate   10 years              
Face amount modification                 $ 75,000,000.0
Unamortized accretion     $ 33,153,000     $ 34,703,000      
Revolving Credit Facility                  
Line of Credit Facility [Abstract]                  
Credit facility borrowing capacity             $ 250,000,000.0    
Collateral ratio       2.50 4.00        
After-acquired property exclusionary restrictions       2.5 4.0        
Covenant, aggregate amount of other investments       $ 50,000,000.0 $ 100,000,000.0        
Number of lenders | lender           4      
Borrowings outstanding     0     $ 0      
Letters of credit secured using cash collateral     0     0      
Letter of Credit, Cash Secured                  
Line of Credit Facility [Abstract]                  
Letters of credit secured using cash collateral     11,200,000     $ 14,100,000      
Standby Letters of Credit | Standby Letters for Credit Facility                  
Line of Credit Facility [Abstract]                  
Credit facility borrowing capacity               $ 50,000,000.0  
Letters of credit secured using cash collateral     $ 37,500,000            
Standby Letters of Credit | Backstop Standby Letters of Credit                  
Line of Credit Facility [Abstract]                  
Credit facility borrowing capacity               $ 40,000,000.0  
Junior Subordinated Notes                  
Junior Subordinated Notes [Abstract]                  
Weighted average fixed interest rate of debt (percent)     4.25%            
Senior Notes | Senior Unsecured Term Loans September 2022                  
Unsecured Debt [Abstract]                  
Aggregate principal amount of debt $ 150,000,000.0                
Periodic payment $ 50,000,000.0                
Stated interest rate on debt instrument (percent) 4.875%