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INVESTMENT SECURITIES - Additional Information (Details)
 3 Months Ended 12 Months Ended
Jun. 30, 2017
USD ($)
Dec. 31, 2017
USD ($)
security
Dec. 31, 2016
USD ($)
security
Dec. 31, 2015
USD ($)
Investments, Debt and Equity Securities [Abstract]        
Held-to maturity securities transferred | $ $ 740,900,000      
Increase to AOCI from transfer | $ $ 22,000,000      
Securities available-for-sale, pledged | $   $ 564,400,000 $ 581,800,000  
Other than temporary impairment losses | $   $ 0 $ 0 $ 0
Securities available-for-sale portfolio | security   191 161  
Securities in unrealized loss position | security   33 59  
Securities held-to-maturity portfolio | security     87  
Held-to-maturity securities in unrealized loss position | security     15