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INVESTMENT SECURITIES - Available-for-Sale Portfolio (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Available-for-Sale Portfolio [Abstract]
Amortized cost	$ 4,039,978	$ 3,427,779
Gross unrealized gains	85,757	136,901
Gross unrealized losses	112,996	16,788
Fair value	4,012,739	3,547,892
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized cost	4,039,978
Fair value	4,012,739
U.S. Treasury and agency securities
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized cost	3,082,187	2,423,236
Gross unrealized gains	52,960	94,741
Gross unrealized losses	110,075	16,595
Fair value	3,025,072	2,501,382
Obligations of state and political subdivisions
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized cost	405,245	451,028
Gross unrealized gains	20,908	24,632
Gross unrealized losses	1	0
Fair value	426,152	475,660
Government agency mortgage-backed securities
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized cost	521,056	506,540
Gross unrealized gains	10,999	16,280
Gross unrealized losses	2,916	182
Fair value	529,139	522,638
Corporate debt securities
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized cost	3,000	4,499
Gross unrealized gains	94	137
Gross unrealized losses	0	1
Fair value	3,094	4,635
Government agency collateralized mortgage obligations
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized cost	28,490	42,476
Gross unrealized gains	796	1,111
Gross unrealized losses	4	10
Fair value	$ 29,282	$ 43,577

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- Definition

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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