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INVESTMENT SECURITIES - Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Available-for-Sale, Debt Maturities, Amortized Cost [Abstract]
Due in one year or less	$ 215,400
Due after one through five years	732,939
Due after five years through ten years	460,678
Due after ten years	1,469,746
Subtotal	2,878,763
Amortized cost	3,427,779
Available-for-Sale, Debt Maturities, Fair Value [Abstract]
Due in one year or less	217,938
Due after one through five years	768,761
Due after five years through ten years	496,977
Due after ten years	1,498,001
Subtotal	2,981,677
Fair value	3,547,892
Government Agency Mortgage-Backed Securities [Member]
Available-for-Sale, Debt Maturities, Amortized Cost [Abstract]
Amortized cost	506,540	$ 428,491
Available-for-Sale, Debt Maturities, Fair Value [Abstract]
Fair value	522,638	432,862
Government Agency Collateralized Mortgage Obligations [Member]
Available-for-Sale, Debt Maturities, Amortized Cost [Abstract]
Amortized cost	42,476	52,621
Available-for-Sale, Debt Maturities, Fair Value [Abstract]
Fair value	43,577	53,071
U.S. Treasury and Agency Securities [Member]
Available-for-Sale, Debt Maturities, Amortized Cost [Abstract]
Amortized cost	2,423,236	2,030,060
Available-for-Sale, Debt Maturities, Fair Value [Abstract]
Fair value	$ 2,501,382	$ 2,043,759

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