SECURITIES AVAILABLE FOR SALE - Securities Impairment (Details) - Available-for-sale Securities - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Estimated credit losses as of period start	$ 1,697	$ 3,180	$ 3,413
Reductions for securities paid off during the period	0	1,483	233
Estimated credit losses at end of the period	$ 1,697	$ 1,697	$ 3,180