INVESTMENT SECURITIES - ROLLFORWARD OF CREDIT LOSSES (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Bank's private label mortgage backed security credit losses recognized in earnings      
Available-for-sale Securities $ 481,275 $ 517,058  
Private label mortgage backed security      
Bank's private label mortgage backed security credit losses recognized in earnings      
Available-for-sale Securities $ 4,777 5,132  
Estimated average life 4 years    
Rollforward of the Bank's private label mortgage backed security credit losses recognized in earnings      
Balance, beginning of period $ 1,765 1,800 $ 1,941
Recovery of losses previously recorded   (35) (141)
Balance, end of period 1,765 $ 1,765 $ 1,800
Additional impairment charge that may be recognized $ 3,700