OFF-BALANCE SHEET ARRANGEMENTS, COMMITMENTS AND CONTINGENCIES - Future Cash Payments Associated With Contractual Obligations (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Certificates and other time deposits
1 year or less	$ 1,075,282
More than 1 year but less than 3 years	244,428
3 years or more but less than 5 years	39,295
Total	1,359,005	$ 1,262,332
Federal Home Loan Bank advances
1 year or less	390,062
More than 1 year but less than 3 years	50,549
Total	440,611
Subordinated debentures and subordinated notes
1 year or less	4,023
More than 1 year but less than 3 years	3,888
3 years or more but less than 5 years	9,345
5 years or more	82,465
Total	99,721
Operating leases
1 year or less	2,143
More than 1 year but less than 3 years	3,377
3 years or more but less than 5 years	2,611
5 years or more	2,283
Total	10,414
Total
1 year or less	1,471,510
More than 1 year but less than 3 years	302,242
3 years or more but less than 5 years	51,251
5 years or more	84,748
Total	1,909,751
Interest payments on Federal Home Loan Bank advances to be paid over future periods.	3,600
Interest included in payments for subordinated debentures	$ 42,600