SECURITIES (Available-for-sale Securities) (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 6,004,596	$ 3,292,998
Amortized Cost, Equity Securities	19,629	18,435
Amortized Cost, Total	6,024,225	3,311,433
Gross Unrealized Gains	127,866	29,765
Gross Unrealized Losses	(24,116)	(28,402)
Securities available for sale	6,108,346	3,294,361
Estimated Fair Value, Equity Securities	19,629	18,435
Estimated Fair Value, Total	6,127,975	3,312,796
U.S. treasuries
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,995	8,466
Gross Unrealized Gains	31	37
Gross Unrealized Losses	0	0
Securities available for sale	2,026	8,503
U.S. agencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	167,048	185,566
Gross Unrealized Gains	657	671
Gross Unrealized Losses	(926)	(1,561)
Securities available for sale	166,779	184,676
Obligations of states and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,562,631	704,073
Gross Unrealized Gains	75,555	12,516
Gross Unrealized Losses	(2,959)	(9,399)
Securities available for sale	1,635,227	707,190
Mortgage-backed securities - agency
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,351,964	763,733
Gross Unrealized Gains	16,029	7,598
Gross Unrealized Losses	(12,723)	(4,605)
Securities available for sale	1,355,270	766,726
Mortgage-backed securities - non-agency
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,428,068	427,315
Gross Unrealized Gains	22,688	4,312
Gross Unrealized Losses	(1,640)	(1,130)
Securities available for sale	1,449,116	430,497
Commercial mortgage-backed securities - agency
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	171,451	68,019
Gross Unrealized Gains	3,440	989
Gross Unrealized Losses	(738)	(143)
Securities available for sale	174,153	68,865
Commercial mortgage-backed securities - non-agency
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	253,421	435,195
Gross Unrealized Gains	37	3,113
Gross Unrealized Losses	(691)	(1,983)
Securities available for sale	252,767	436,325
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,064,255	700,631
Gross Unrealized Gains	9,421	529
Gross Unrealized Losses	(4,410)	(9,581)
Securities available for sale	1,069,266	691,579
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	3,763	0
Gross Unrealized Gains	8	0
Gross Unrealized Losses	(29)	0
Securities available for sale	$ 3,742	$ 0

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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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