INVESTMENTS (Summary of Amortized Costs and Estimated Fair Values of Available-For-Sale Securities) (Details) - USD ($) $ in Thousands	Mar. 31, 2022	Dec. 31, 2021
Debt Securities, Available-for-sale [Abstract]
Amortized Cost, Total	$ 1,516,057	$ 1,508,981
Available-for-sale securities, at fair value (amortized cost of $1,516,057 and $1,508,981, respectively)	1,421,809	1,507,486
Obligations of U.S. government-sponsored enterprises
Debt Securities, Available-for-sale [Abstract]
Amortized Cost, Total	8,585	8,585
Unrealized Gains	0	0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	915	241
Available-for-sale securities, at fair value (amortized cost of $1,516,057 and $1,508,981, respectively)	7,670	8,344
Obligations of states and political subdivisions
Debt Securities, Available-for-sale [Abstract]
Amortized Cost, Total	113,388	112,086
Unrealized Gains	587	5,392
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	2,019	0
Available-for-sale securities, at fair value (amortized cost of $1,516,057 and $1,508,981, respectively)	111,956	117,478
Mortgage-backed securities issued or guaranteed by U.S. government-sponsored enterprises
Debt Securities, Available-for-sale [Abstract]
Amortized Cost, Total	1,029,454	1,003,869
Unrealized Gains	435	7,856
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	69,051	11,468
Available-for-sale securities, at fair value (amortized cost of $1,516,057 and $1,508,981, respectively)	960,838	1,000,257
Collateralized mortgage obligations issued or guaranteed by U.S. government-sponsored enterprises
Debt Securities, Available-for-sale [Abstract]
Amortized Cost, Total	341,998	361,781
Unrealized Gains	145	2,835
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	22,252	5,767
Available-for-sale securities, at fair value (amortized cost of $1,516,057 and $1,508,981, respectively)	319,891	358,849
Subordinated corporate bonds
Debt Securities, Available-for-sale [Abstract]
Amortized Cost, Total	22,632	22,660
Unrealized Gains	55	152
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	1,233	254
Available-for-sale securities, at fair value (amortized cost of $1,516,057 and $1,508,981, respectively)	21,454	22,558
Total AFS debt securities
Debt Securities, Available-for-sale [Abstract]
Amortized Cost, Total	1,516,057	1,508,981
Unrealized Gains	1,222	16,235
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	95,470	17,730
Available-for-sale securities, at fair value (amortized cost of $1,516,057 and $1,508,981, respectively)	$ 1,421,809	$ 1,507,486