INVESTMENT SECURITIES - Amortized cost and estimated fair value of debt securities by contractual maturity (Details) - USD ($) $ in Thousands	Dec. 31, 2022	Dec. 31, 2021
Amortized Cost
Due in one year or less	$ 521,444
Due after one through five years	1,728,988
Due after five years through ten years	1,161,641
Due after ten years	805,591
Subtotal	4,217,664
Total - Amortized cost.	4,675,474	$ 4,980,102
Fair Value
Due in one year or less	516,496
Due after one through five years	1,574,685
Due after five years through ten years	1,020,562
Due after ten years	643,205
Subtotal	3,754,948
Total - Fair value	4,151,851	4,934,210
Amortized Cost
Due in one year or less	0
Due after one through five years	0
Due after five years through ten years	539,825
Due after ten years	539,870
Subtotal	1,079,695
Total - Amortized Cost	1,079,695	0
Fair Value
Due in one year or less	0
Due after one through five years	0
Due after five years through ten years	524,815
Due after ten years	509,980
Subtotal	1,034,795
Total - Fair Value	1,034,795	0
Government agency mortgage-backed securities
Amortized Cost
Total - Amortized cost.	444,689	474,506
Fair Value
Total - Fair value	384,633	477,056
Amortized Cost
Total - Amortized Cost	0
Fair Value
Total - Fair Value	0
Government agency collateralized mortgage obligations
Amortized Cost
Total - Amortized cost.	13,121	19,953
Fair Value
Total - Fair value	12,270	20,339
Amortized Cost
Total - Amortized Cost	0
Fair Value
Total - Fair Value	0
US Treasury And Government
Amortized Cost
Total - Amortized cost.	3,660,546	4,064,624
Fair Value
Total - Fair value	3,243,537	3,998,564
Amortized Cost
Total - Amortized Cost	1,079,695	0
Fair Value
Total - Fair Value	$ 1,034,795	$ 0