INVESTMENT SECURITIES - Amortized cost and estimated fair value of debt securities by contractual maturity (Details) - USD ($) $ in Thousands	Jun. 30, 2023	Dec. 31, 2022
Available-for-Sale, Debt Maturities, Amortized Cost
Due in one year or less	$ 170,995
Due after one through five years	1,013,648
Due after five years through ten years	1,100,527
Due after ten years	781,766
Subtotal	3,066,936
Amortized cost	3,500,726	$ 4,675,474
Available-for-Sale, Fair Value
Due in one year or less	170,364
Due after one through five years	909,908
Due after five years through ten years	972,563
Due after ten years	636,070
Subtotal	2,688,905
Fair value	3,064,870	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	546,275
Due after ten years	547,992
Government agency mortgage-backed securities	1,104,020	1,079,695
Subtotal	1,094,267
Total - Amortized Cost	1,104,020
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	529,303
Due after ten years	524,510
Subtotal	1,053,813
Total - Fair Value	1,063,444	1,034,795
Government agency mortgage-backed securities
Available-for-Sale, Debt Maturities, Amortized Cost
Amortized cost	422,521	444,689
Available-for-Sale, Fair Value
Fair value	365,506	384,633
Held-to-Maturity, Amortized Cost
Government agency mortgage-backed securities	9,753
Held-to-Maturity, Fair Value
Total - Fair Value	9,631	0
Government agency collateralized mortgage obligations
Available-for-Sale, Debt Maturities, Amortized Cost
Amortized cost	11,269	13,121
Available-for-Sale, Fair Value
Fair value	10,459	12,270
Held-to-Maturity, Amortized Cost
Total - Amortized Cost	0
Held-to-Maturity, Fair Value
Total - Fair Value	0
U.S. Treasury Securities
Available-for-Sale, Debt Maturities, Amortized Cost
Amortized cost	2,529,014	3,660,546
Available-for-Sale, Fair Value
Fair value	2,189,845	3,243,537
Held-to-Maturity, Fair Value
Total - Fair Value	$ 1,053,813	$ 1,034,795