SECURITIES (Securities available for sale) (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Gross Unrealized Gains	$ 92,661	$ 127,866
Gross Unrealized Losses	(79,748)	(24,116)
Equity securities with a readily determinable fair value	20,543	19,629
Amortized, Total	6,357,582	6,024,225
Estimated Fair Value, Total	6,370,495	6,127,975
Total debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	6,337,039	6,004,596
Gross Unrealized Gains	92,661	127,866
Gross Unrealized Losses	(79,748)	(24,116)
Estimated Fair Value	6,349,952	6,108,346
U.S. treasuries
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	995	1,995
Gross Unrealized Gains	24	31
Gross Unrealized Losses	0	0
Estimated Fair Value	1,019	2,026
U.S. agencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	212,805	167,048
Gross Unrealized Gains	1,222	657
Gross Unrealized Losses	(1,735)	(926)
Estimated Fair Value	212,292	166,779
Obligations of states and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,737,638	1,562,631
Gross Unrealized Gains	41,460	75,555
Gross Unrealized Losses	(41,109)	(2,959)
Estimated Fair Value	1,737,989	1,635,227
Mortgage-backed securities - agency
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,442,422	1,351,964
Gross Unrealized Gains	19,675	16,029
Gross Unrealized Losses	(29,168)	(12,723)
Estimated Fair Value	1,432,929	1,355,270
Mortgage-backed securities - non-agency
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,490,868	1,428,068
Gross Unrealized Gains	14,957	22,688
Gross Unrealized Losses	(2,109)	(1,640)
Estimated Fair Value	1,503,716	1,449,116
Commercial mortgage-backed securities - agency
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	123,797	171,451
Gross Unrealized Gains	2,150	3,440
Gross Unrealized Losses	(3,717)	(738)
Estimated Fair Value	122,230	174,153
Commercial mortgage-backed securities - non-agency
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	307,744	253,421
Gross Unrealized Gains	272	37
Gross Unrealized Losses	(242)	(691)
Estimated Fair Value	307,774	252,767
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,017,016	1,064,255
Gross Unrealized Gains	12,837	9,421
Gross Unrealized Losses	(1,640)	(4,410)
Estimated Fair Value	1,028,213	1,069,266
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	3,754	3,763
Gross Unrealized Gains	64	8
Gross Unrealized Losses	(28)	(29)
Estimated Fair Value	$ 3,790	$ 3,742

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