INVESTMENT SECURITIES - Summary of Unrealized Losses - Available-for-Sale (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Securities with losses under 12 months, Fair Value	$ 93,037	$ 232,776
Securities with losses under 12 months, Gross Unrealized Loss	826	4,466
Securities with losses over 12 months, Fair Value	158,412	16,315
Securities with losses over 12 months, Gross Unrealized Loss	4,528	556
Total, Fair Value	251,449	249,091
Total, Gross Unrealized Loss	5,354	5,022
Obligations of state and political subdivisions
Schedule of Available-for-sale Securities [Line Items]
Securities with losses under 12 months, Fair Value	596	13,402
Securities with losses under 12 months, Gross Unrealized Loss	5	1,020
Securities with losses over 12 months, Fair Value	12,716	0
Securities with losses over 12 months, Gross Unrealized Loss	437	0
Total, Fair Value	13,312	13,402
Total, Gross Unrealized Loss	442	1,020
GSE mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Securities with losses under 12 months, Fair Value	29,725	29,119
Securities with losses under 12 months, Gross Unrealized Loss	224	302
Securities with losses over 12 months, Fair Value	5,858	0
Securities with losses over 12 months, Gross Unrealized Loss	97	0
Total, Fair Value	35,583	29,119
Total, Gross Unrealized Loss	321	302
Collateralized mortgage obligations: residential
Schedule of Available-for-sale Securities [Line Items]
Securities with losses under 12 months, Fair Value	57,665	187,235
Securities with losses under 12 months, Gross Unrealized Loss	548	3,099
Securities with losses over 12 months, Fair Value	137,598	14,194
Securities with losses over 12 months, Gross Unrealized Loss	3,960	507
Total, Fair Value	195,263	201,429
Total, Gross Unrealized Loss	4,508	3,606
Collateralized mortgage obligations: commercial
Schedule of Available-for-sale Securities [Line Items]
Securities with losses under 12 months, Fair Value	0	961
Securities with losses under 12 months, Gross Unrealized Loss	0	4
Securities with losses over 12 months, Fair Value	2,240	2,121
Securities with losses over 12 months, Gross Unrealized Loss	34	49
Total, Fair Value	2,240	3,082
Total, Gross Unrealized Loss	34	53
Mutual funds
Schedule of Available-for-sale Securities [Line Items]
Securities with losses under 12 months, Fair Value	2,061	2,059
Securities with losses under 12 months, Gross Unrealized Loss	39	41
Securities with losses over 12 months, Fair Value	0	0
Securities with losses over 12 months, Gross Unrealized Loss	0	0
Total, Fair Value	2,061	2,059
Total, Gross Unrealized Loss	39	$ 41
Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Securities with losses under 12 months, Fair Value	2,990
Securities with losses under 12 months, Gross Unrealized Loss	10
Total, Fair Value	2,990
Total, Gross Unrealized Loss	$ 10