v3.8.0.1
Borrowings - Additional Information (Details)
12 Months Ended
Dec. 31, 2017
USD ($)
Institution
Dec. 31, 2016
USD ($)
Jan. 31, 2018
Jan. 31, 2017
Debt Instrument [Line Items]        
Overnight FHLB advances $ 628,000,000 $ 1,300,000,000    
Long-term line of credit 0      
FHLB advances interest rates       0.56%
Loans pledged as collateral 3,000,000,000      
Federal Home Loan Bank, maximum borrowing capacity 2,400,000,000      
Federal Home Loan Bank, additional credit available 168,000,000      
Unsecured debt $ 120,000,000      
Number of financial institutions | Institution 5      
Federal Home Loan Bank unused lines of credit $ 943,100,000      
FHLB, average daily balance of borrowings outstanding 519,000,000 $ 507,000,000    
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      
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 $ 50,000,000      
Line of credit facility, Interest rate 4.83%      
Subsequent Event        
Debt Instrument [Line Items]        
FHLB advances interest rates     1.41%