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FAIR VALUE - Financial assets and liabilities accounted for at fair value on a recurring basis (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2023	Dec. 31, 2022
Available-for-sale investment securities
Fair Value	$ 2,767,636	$ 4,151,851
Equity securities	320	419
Mortgage loans held for sale		0
Unpaid principal value of mortgage loans held for sale	600
Transfers between Level 1 and Level 2	0
Transfers between Level 2 and Level 1	0
Transfers in of level 3	0
Transfers Out of level 3	0
U.S. Treasury and agency securities
Available-for-sale investment securities
Fair Value	1,979,439	3,243,537
Obligations of state and political subdivisions
Available-for-sale investment securities
Fair Value	434,465	504,297
Government agency mortgage-backed securities
Available-for-sale investment securities
Fair Value	337,071	384,633
Corporate debt securities
Available-for-sale investment securities
Fair Value	7,174	7,114
Government agency collateralized mortgage obligations
Available-for-sale investment securities
Fair Value	9,487	12,270
Recurring
Available-for-sale investment securities
Fair Value	2,767,636	4,151,851
Equity securities	320	419
Mortgage loans held for sale	604
Total	2,768,583	4,152,280
Recurring | Interest rate swap agreements
Available-for-sale investment securities
Derivative asset		1
Derivative liability		(1)
Recurring | Forward sales commitments
Available-for-sale investment securities
Derivative asset	10	5
Recurring | Commitments to originate real estate loans for sale
Available-for-sale investment securities
Derivative asset	13	5
Recurring | U.S. Treasury and agency securities
Available-for-sale investment securities
Fair Value	1,979,439	3,243,537
Recurring | Obligations of state and political subdivisions
Available-for-sale investment securities
Fair Value	434,465	504,297
Recurring | Government agency mortgage-backed securities
Available-for-sale investment securities
Fair Value	337,071	384,633
Recurring | Corporate debt securities
Available-for-sale investment securities
Fair Value	7,174	7,114
Recurring | Government agency collateralized mortgage obligations
Available-for-sale investment securities
Fair Value	9,487	12,270
Recurring | Level 1
Available-for-sale investment securities
Fair Value	1,921,264	3,178,189
Equity securities	320	419
Mortgage loans held for sale	0
Total	1,921,584	3,178,608
Recurring | Level 1 | Interest rate swap agreements
Available-for-sale investment securities
Derivative asset		0
Derivative liability		0
Recurring | Level 1 | Forward sales commitments
Available-for-sale investment securities
Derivative asset	0	0
Recurring | Level 1 | Commitments to originate real estate loans for sale
Available-for-sale investment securities
Derivative asset	0	0
Recurring | Level 1 | U.S. Treasury and agency securities
Available-for-sale investment securities
Fair Value	1,921,264	3,178,189
Recurring | Level 1 | Obligations of state and political subdivisions
Available-for-sale investment securities
Fair Value	0	0
Recurring | Level 1 | Government agency mortgage-backed securities
Available-for-sale investment securities
Fair Value	0	0
Recurring | Level 1 | Corporate debt securities
Available-for-sale investment securities
Fair Value	0	0
Recurring | Level 1 | Government agency collateralized mortgage obligations
Available-for-sale investment securities
Fair Value	0	0
Recurring | Level 2
Available-for-sale investment securities
Fair Value	846,372	973,662
Equity securities	0	0
Mortgage loans held for sale	604
Total	846,986	973,667
Recurring | Level 2 | Interest rate swap agreements
Available-for-sale investment securities
Derivative asset		1
Derivative liability		(1)
Recurring | Level 2 | Forward sales commitments
Available-for-sale investment securities
Derivative asset	10	5
Recurring | Level 2 | Commitments to originate real estate loans for sale
Available-for-sale investment securities
Derivative asset	0	0
Recurring | Level 2 | U.S. Treasury and agency securities
Available-for-sale investment securities
Fair Value	58,175	65,348
Recurring | Level 2 | Obligations of state and political subdivisions
Available-for-sale investment securities
Fair Value	434,465	504,297
Recurring | Level 2 | Government agency mortgage-backed securities
Available-for-sale investment securities
Fair Value	337,071	384,633
Recurring | Level 2 | Corporate debt securities
Available-for-sale investment securities
Fair Value	7,174	7,114
Recurring | Level 2 | Government agency collateralized mortgage obligations
Available-for-sale investment securities
Fair Value	9,487	12,270
Recurring | Level 3
Available-for-sale investment securities
Fair Value	0	0
Equity securities	0	0
Mortgage loans held for sale	0
Total	13	5
Recurring | Level 3 | Interest rate swap agreements
Available-for-sale investment securities
Derivative asset		0
Derivative liability		0
Recurring | Level 3 | Forward sales commitments
Available-for-sale investment securities
Derivative asset	0	0
Recurring | Level 3 | Commitments to originate real estate loans for sale
Available-for-sale investment securities
Derivative asset	13	5
Recurring | Level 3 | U.S. Treasury and agency securities
Available-for-sale investment securities
Fair Value	0	0
Recurring | Level 3 | Obligations of state and political subdivisions
Available-for-sale investment securities
Fair Value	0	0
Recurring | Level 3 | Government agency mortgage-backed securities
Available-for-sale investment securities
Fair Value	0	0
Recurring | Level 3 | Corporate debt securities
Available-for-sale investment securities
Fair Value	0	0
Recurring | Level 3 | Government agency collateralized mortgage obligations
Available-for-sale investment securities
Fair Value	$ 0	$ 0

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- Definition

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+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 820
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (c)(3)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482106/820-10-50-2

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- Definition

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+ References

No definition available.

+ Details

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- Definition

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+ References

No definition available.

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