INVESTMENT SECURITIES - Amortized cost and estimated fair value of debt securities by contractual maturity (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Available-for-Sale, Debt Maturities, Amortized Cost
Due in one year or less	$ 17,031
Due after one through five years	1,473,904
Due after five years through ten years	685,913
Due after ten years	716,597
Subtotal	2,893,445
Amortized Cost	3,314,815	$ 4,675,474
Available-for-Sale, Fair Value
Due in one year or less	16,799
Due after one through five years	1,295,411
Due after five years through ten years	587,185
Due after ten years	521,683
Subtotal	2,421,078
Fair Value	2,767,636	4,151,851
Held-to-Maturity, Amortized Cost
Due in one year or less	0
Due after one through five years	0
Due after five years through ten years	549,555
Due after ten years	552,124
Subtotal	1,101,679
Government agency mortgage-backed securities	1,125,540	1,079,695
Amortized Cost	1,125,540
Held-to-Maturity, Fair Value
Due in one year or less	0
Due after one through five years	0
Due after five years through ten years	506,275
Due after ten years	461,313
Subtotal	967,588
Fair Value	990,660	1,034,795
Government agency mortgage-backed securities
Available-for-Sale, Debt Maturities, Amortized Cost
Amortized Cost	411,083	444,689
Available-for-Sale, Fair Value
Fair Value	337,071	384,633
Held-to-Maturity, Amortized Cost
Government agency mortgage-backed securities	23,861
Held-to-Maturity, Fair Value
Fair Value	23,072	0
Government agency collateralized mortgage obligations
Available-for-Sale, Debt Maturities, Amortized Cost
Amortized Cost	10,287	13,121
Available-for-Sale, Fair Value
Fair Value	9,487	$ 12,270
Held-to-Maturity, Amortized Cost
Amortized Cost	0
Held-to-Maturity, Fair Value
Fair Value	$ 0