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Debt - Narrative (Details) - USD ($)
3 Months Ended 9 Months Ended 12 Months Ended
Dec. 07, 2017
Nov. 23, 2015
Dec. 31, 2017
Sep. 30, 2019
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Jun. 21, 2019
Jun. 20, 2019
Debt Instrument [Line Items]                  
Purchase of FHLB activity-based common stock as percentage of borrowing, required         4.50%        
Purchase of FHLB activity-based common stock as percentage of borrowing, percentage, authorized         15.00%        
Principal borrowings on senior revolving credit facility $ 25,000,000   $ 50,000,000.0   $ 135,000,000 $ 0 $ 0    
FHLB advances interest rate         1.99%        
Line of Credit                  
Debt Instrument [Line Items]                  
Debt term       5 years          
Borrowing capacity               $ 150,000,000.0 $ 225,000,000.0
Senior revolving credit facility         $ 135,000,000.0        
Commitment fee percent         0.15%        
4.50% Senior Notes                  
Debt Instrument [Line Items]                  
Principal amount   $ 250,000,000.0     $ 250,000,000        
Stated rate   4.50%     4.50%        
Debt redemption price, percentage   100.00%              
Debt redemption price, interest discounted at treasury yield margin   0.35%              
Discount percent on issuance   0.265%              
Effective interest percentage   4.53%              
October 5, 2022                  
Debt Instrument [Line Items]                  
FHLB Advances     25,000,000.0       25,000,000.0    
December 2, 2022                  
Debt Instrument [Line Items]                  
FHLB Advances     $ 25,000,000.0       $ 25,000,000.0    
Long-term debt                  
Debt Instrument [Line Items]                  
FHLB Advances         $ 50,000,000.0        
London Interbank Offered Rate (LIBOR) | Line of Credit                  
Debt Instrument [Line Items]                  
Basis spread on variable rate         1.15%