v3.8.0.1
Borrowings - Additional Information (Details)
3 Months Ended 12 Months Ended
Mar. 31, 2018
USD ($)
Institution
Dec. 31, 2017
USD ($)
Apr. 30, 2018
USD ($)
Jan. 31, 2018
Debt Instrument [Line Items]        
Overnight FHLB advances $ 739,000,000 $ 628,000,000    
Long-term line of credit 0      
FHLB advances interest rates       1.41%
Loans pledged as collateral 3,200,000,000      
Federal Home Loan Bank, maximum borrowing capacity 1,800,000,000      
Federal Home Loan Bank, additional credit available 169,000,000      
Unsecured debt $ 120,000,000      
Number of financial institutions | Institution 5      
Federal Home Loan Bank unused lines of credit $ 1,000,000 943,000,000    
FHLB, average daily balance of borrowings outstanding 693,000,000 499,000,000    
Minimum        
Debt Instrument [Line Items]        
Unsecured debt 20,000,000      
Maximum        
Debt Instrument [Line Items]        
Unsecured debt 25,000,000      
Revolving Credit Facility        
Debt Instrument [Line Items]        
Line of credit, borrowing capacity   $ 50,000,000    
Line of credit maturity period   5 years    
Line of credit, current borrowing capacity 25,000,000      
Revolving Credit Facility | 90 Day LIBOR        
Debt Instrument [Line Items]        
Debt instrument basis point rate   3.50%    
FFI        
Debt Instrument [Line Items]        
Long-term line of credit $ 30,000,000 $ 50,000,000    
Line of credit facility, Interest rate 5.19%      
Subsequent Event        
Debt Instrument [Line Items]        
FHLB advances interest rates     1.87%  
Subsequent Event | Revolving Credit Facility        
Debt Instrument [Line Items]        
Line of credit, current borrowing capacity     $ 75,000,000