INVESTMENTS (Summary of Amortized Costs and Estimated Fair Values of Available-For-Sale Securities) (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Abstract]
Amortized Cost, Total	$ 1,443,800	$ 1,078,474
Available-for-sale securities, at fair value (book value of $1,443,800 and $1,078,474, respectively)	1,455,210	1,115,813
Obligations of U.S. government-sponsored enterprises
Debt Securities, Available-for-sale [Abstract]
Amortized Cost, Total	8,585
Unrealized Gains	1
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	134
Available-for-sale securities, at fair value (book value of $1,443,800 and $1,078,474, respectively)	8,452
Obligations of states and political subdivisions
Debt Securities, Available-for-sale [Abstract]
Amortized Cost, Total	112,688	119,608
Unrealized Gains	5,267	7,627
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	202	115
Available-for-sale securities, at fair value (book value of $1,443,800 and $1,078,474, respectively)	117,753	127,120
Mortgage-backed securities issued or guaranteed by U.S. government-sponsored enterprises
Debt Securities, Available-for-sale [Abstract]
Amortized Cost, Total	915,317	547,396
Unrealized Gains	11,286	19,796
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	6,611	574
Available-for-sale securities, at fair value (book value of $1,443,800 and $1,078,474, respectively)	919,992	566,618
Collateralized mortgage obligations issued or guaranteed by U.S. government-sponsored enterprises
Debt Securities, Available-for-sale [Abstract]
Amortized Cost, Total	391,128	399,937
Unrealized Gains	4,595	10,652
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	2,896	135
Available-for-sale securities, at fair value (book value of $1,443,800 and $1,078,474, respectively)	392,827	410,454
Subordinated corporate bonds
Debt Securities, Available-for-sale [Abstract]
Amortized Cost, Total	16,082	11,533
Unrealized Gains	205	186
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	101	98
Available-for-sale securities, at fair value (book value of $1,443,800 and $1,078,474, respectively)	16,186	11,621
Total AFS debt securities
Debt Securities, Available-for-sale [Abstract]
Amortized Cost, Total	1,443,800	1,078,474
Unrealized Gains	21,354	38,261
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	9,944	922
Available-for-sale securities, at fair value (book value of $1,443,800 and $1,078,474, respectively)	$ 1,455,210	$ 1,115,813

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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Amount, before tax, of unrealized loss 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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Amortized cost of 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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Amount of 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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