BORROWINGS AND BORROWING CAPACITY - Narrative (Details) - USD ($)
1 Months Ended 12 Months Ended
Sep. 30, 2015
Oct. 31, 2015
Dec. 31, 2016
Dec. 31, 2015
Debt Instrument [Line Items]        
Increase (decrease) in other borrowings     $ 18,000,000 $ 10,100,000
Repayments of debt   $ 27,500,000    
Borrowing Agreement | Prime Rate        
Debt Instrument [Line Items]        
Basis spread on variable rate 0.25%      
Interest rate, effective percentage 3.50%      
Federal Home Loan Bank of Dallas        
Debt Instrument [Line Items]        
Borrowing capacity     683,100,000.0  
Remaining borrowing capacity     362,700,000  
Outstanding amount     320,400,000  
Letters of credit outstanding, amount     35,400,000  
Federal Home Loan Bank of Dallas | Expire in February 2017        
Debt Instrument [Line Items]        
Letters of credit outstanding, amount     3,000,000  
Federal Home Loan Bank of Dallas | Expire in August 2017        
Debt Instrument [Line Items]        
Letters of credit outstanding, amount     7,300,000  
Federal Home Loan Bank of Dallas | Expire in October 2017        
Debt Instrument [Line Items]        
Letters of credit outstanding, amount     25,000,000  
Federal Home Loan Bank of Dallas | Short-term Debt        
Debt Instrument [Line Items]        
Federal Home Loan Bank, amount of advances     $ 85,000,000  
Federal Home Loan Bank of Dallas | Short-term Debt | Weighted Average        
Debt Instrument [Line Items]        
Federal Home Loan Bank, interest rate     0.55%  
Federal Home Loan Bank of Dallas | Long-term Debt        
Debt Instrument [Line Items]        
Federal Home Loan Bank, amount of advances     $ 200,000,000  
Federal Home Loan Bank of Dallas | Long-term Debt | Expire in April 2018        
Debt Instrument [Line Items]        
Federal Home Loan Bank, amount of advances     100,000,000  
Federal Home Loan Bank of Dallas | Long-term Debt | Expire in May 2018        
Debt Instrument [Line Items]        
Federal Home Loan Bank, amount of advances     $ 100,000,000  
Federal Home Loan Bank of Dallas | Long-term Debt | Weighted Average        
Debt Instrument [Line Items]        
Federal Home Loan Bank, interest rate     0.56%