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INVESTMENT SECURITIES, Available-for-Sale Portfolio (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Available-for-Sale Portfolio
Amortized cost	$ 3,068,406	$ 3,011,551
Gross unrealized gains	154,602	42,017
Gross unrealized losses	286	9,140
Fair value	3,222,722	3,044,428
Debt Securities, Available-for-sale, Fair Value to Amortized Cost
Amortized cost	3,068,406
Fair value	3,222,722
U.S. Treasury Securities [Member]
Debt Securities, Available-for-sale, Fair Value to Amortized Cost
Amortized cost	2,034,512	2,030,060
Gross unrealized gains	114,658	21,674
Gross unrealized losses	0	7,975
Fair value	2,149,170	2,043,759
Obligations of State and Political Subdivisions [Member]
Debt Securities, Available-for-sale, Fair Value to Amortized Cost
Amortized cost	470,518	497,852
Gross unrealized gains	22,978	14,382
Gross unrealized losses	0	26
Fair value	493,496	512,208
Government Agency Mortgage-Backed Securities [Member]
Debt Securities, Available-for-sale, Fair Value to Amortized Cost
Amortized cost	508,738	428,491
Gross unrealized gains	15,514	5,478
Gross unrealized losses	278	1,107
Fair value	523,974	432,862
Corporate Debt Securities [Member]
Debt Securities, Available-for-sale, Fair Value to Amortized Cost
Amortized cost	4,504	2,527
Gross unrealized gains	98	1
Gross unrealized losses	0	0
Fair value	4,602	2,528
Government Agency Collateralized Mortgage Obligations [Member]
Debt Securities, Available-for-sale, Fair Value to Amortized Cost
Amortized cost	50,134	52,621
Gross unrealized gains	1,354	482
Gross unrealized losses	8	32
Fair value	$ 51,480	$ 53,071

X

- 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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