OFF-BALANCE SHEET ARRANGEMENTS, COMMITMENTS AND CONTINGENCIES - Summary of Contractual Obligations (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Certificates and other time deposits
1 year or less	$ 889,051
More than 1 year but less than 3 years	330,780
3 years or more but less than 5 years	42,501
Total	1,262,332	$ 1,365,449
Federal Home Loan Bank advances
1 year or less	325,692
Total	325,692
Subordinated debentures and subordinated notes
1 year or less	3,859
More than 1 year but less than 3 years	3,735
3 years or more but less than 5 years	8,099
5 years or more	82,750
Total	98,443
Operating leases
1 year or less	2,033
More than 1 year but less than 3 years	2,847
3 years or more but less than 5 years	1,916
5 years or more	2,911
Total	9,707
Total
1 year or less	1,220,635
More than 1 year but less than 3 years	337,362
3 years or more but less than 5 years	52,516
5 years or more	85,661
Total	1,696,174
Interest on advancements	692
Interest included in payments for subordinated debentures	$ 41,300