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Borrowings - Narrative (Details)
Sep. 09, 2019
USD ($)
Jul. 30, 2016
Dec. 31, 2020
USD ($)
lender
Oct. 08, 2020
USD ($)
Sep. 30, 2020
USD ($)
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   7.987%          
Junior Subordinated Notes [Abstract]              
Effective period of debt instrument interest rate   10 years          
Face amount modification             $ 75,000,000.0
Unamortized accretion     $ 32,120,000   $ 32,636,000    
Revolving Credit Facility              
Line of Credit Facility [Abstract]              
Credit facility borrowing capacity     $ 250,000,000.0        
Number of lenders | lender     4        
Covenant, aggregate amount of other investments       $ 300,000,000.0      
Minimum liquidity covenant, interest coverage ratio       1.00      
Minimum housing collateral ratio       1.75      
Minimum liquidity, amount       $ 50,000,000.0      
Minimum liquidity, outside of initial requirements, amount       $ 100,000,000.0      
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     11,700,000   $ 12,700,000    
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     $ 25,700,000        
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     4.25%        
Senior Notes | Senior Unsecured Term Loans September 2022              
Unsecured Debt [Abstract]              
Aggregate principal amount of debt $ 100,000,000.0            
Periodic payment $ 50,000,000.0            
Stated interest rate on debt instrument 4.875%