v3.8.0.1
Borrowings - Additional Information (Details) - USD ($)
$ in Millions
9 Months Ended 12 Months Ended
Sep. 30, 2017
Dec. 31, 2016
Oct. 31, 2017
Jun. 30, 2017
Mar. 31, 2017
Jan. 31, 2017
Debt Instrument [Line Items]            
Overnight FHLB advances $ 406.0 $ 1,300.0        
Long-term line of credit 15.0          
FHLB advances interest rates           0.56%
FHLB, average balance of overnight borrowings $ 549.4 $ 507.0        
Line of credit facility, Interest rate 4.80%          
Revolving Credit Facility            
Debt Instrument [Line Items]            
Line of credit, borrowing capacity       $ 50.0 $ 25.0  
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 $ 15.0          
Subsequent Event            
Debt Instrument [Line Items]            
FHLB advances interest rates     1.14%