INVESTMENT SECURITIES - ROLLFORWARD OF CREDIT LOSSES (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Bank's private label mortgage backed security credit losses recognized in earnings
Available-for-sale Securities	$ 527,231	$ 481,275
Number of securities held	185	179
Private label mortgage backed security
Bank's private label mortgage backed security credit losses recognized in earnings
Available-for-sale Securities	$ 4,449	$ 4,777
Number of securities held	1
Rollforward of the Bank's private label mortgage backed security credit losses recognized in earnings
Balance, beginning of period	$ 1,765	1,765	$ 1,800
Recovery of losses previously recorded	0	0	(35)
Balance, end of period	1,765	$ 1,765	$ 1,765
Additional impairment charge that may be recognized	$ 3,100