SECURITIES (Debt securities available for sale) (Details) $ in Thousands	Sep. 30, 2021 USD ($) security	Dec. 31, 2020 USD ($) security
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than 12 months	$ 2,926,498	$ 1,672,530
Unrealized Losses, Less than 12 months	$ (46,639)	$ (18,373)
Count, Less than 12 months | security	228	127
Fair Value, 12 months or longer	$ 469,345	$ 508,436
Unrealized Losses, 12 months or longer	$ (14,523)	$ (5,743)
Count, 12 months or longer | security	43	103
Fair Value, Total	$ 3,395,843	$ 2,180,966
Unrealized Losses, Total	$ (61,162)	$ (24,116)
Count, Total | security	271	230
U.S. agencies
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than 12 months	$ 18,861	$ 2,981
Unrealized Losses, Less than 12 months	$ (628)	$ (8)
Count, Less than 12 months | security	1	5
Fair Value, 12 months or longer	$ 0	$ 99,922
Unrealized Losses, 12 months or longer	$ 0	$ (918)
Count, 12 months or longer | security	0	72
Fair Value, Total	$ 18,861	$ 102,903
Unrealized Losses, Total	$ (628)	$ (926)
Count, Total | security	1	77
Obligations of states and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than 12 months	$ 740,885	$ 346,598
Unrealized Losses, Less than 12 months	$ (18,939)	$ (2,959)
Count, Less than 12 months | security	131	49
Fair Value, 12 months or longer	$ 47,453	$ 0
Unrealized Losses, 12 months or longer	$ (2,169)	$ 0
Count, 12 months or longer | security	13	0
Fair Value, Total	$ 788,338	$ 346,598
Unrealized Losses, Total	$ (21,108)	$ (2,959)
Count, Total | security	144	49
Mortgage-backed securities - agency
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than 12 months	$ 1,413,527	$ 653,793
Unrealized Losses, Less than 12 months	$ (21,898)	$ (12,342)
Count, Less than 12 months | security	58	35
Fair Value, 12 months or longer	$ 336,528	$ 31,012
Unrealized Losses, 12 months or longer	$ (11,734)	$ (381)
Count, 12 months or longer | security	15	3
Fair Value, Total	$ 1,750,055	$ 684,805
Unrealized Losses, Total	$ (33,632)	$ (12,723)
Count, Total | security	73	38
Mortgage-backed securities - non-agency
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than 12 months	$ 468,119	$ 378,843
Unrealized Losses, Less than 12 months	$ (1,241)	$ (1,639)
Count, Less than 12 months | security	12	17
Fair Value, 12 months or longer	$ 48,728	$ 1,622
Unrealized Losses, 12 months or longer	$ (252)	$ (1)
Count, 12 months or longer | security	5	1
Fair Value, Total	$ 516,847	$ 380,465
Unrealized Losses, Total	$ (1,493)	$ (1,640)
Count, Total | security	17	18
Commercial mortgage-backed securities - agency
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than 12 months	$ 80,595	$ 46,541
Unrealized Losses, Less than 12 months	$ (2,769)	$ (738)
Count, Less than 12 months | security	14	6
Fair Value, 12 months or longer	$ 0	$ 0
Unrealized Losses, 12 months or longer	$ 0	$ 0
Count, 12 months or longer | security	0	0
Fair Value, Total	$ 80,595	$ 46,541
Unrealized Losses, Total	$ (2,769)	$ (738)
Count, Total | security	14	6
Commercial mortgage-backed securities - non-agency
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than 12 months	$ 76,559	$ 100,042
Unrealized Losses, Less than 12 months	$ (218)	$ (15)
Count, Less than 12 months | security	1	2
Fair Value, 12 months or longer	$ 14,944	$ 35,428
Unrealized Losses, 12 months or longer	$ (172)	$ (676)
Count, 12 months or longer | security	2	3
Fair Value, Total	$ 91,503	$ 135,470
Unrealized Losses, Total	$ (390)	$ (691)
Count, Total | security	3	5
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than 12 months	$ 127,455	$ 141,824
Unrealized Losses, Less than 12 months	$ (943)	$ (643)
Count, Less than 12 months | security	10	9
Fair Value, 12 months or longer	$ 21,692	$ 340,452
Unrealized Losses, 12 months or longer	$ (196)	$ (3,767)
Count, 12 months or longer | security	8	24
Fair Value, Total	$ 149,147	$ 482,276
Unrealized Losses, Total	$ (1,139)	$ (4,410)
Count, Total | security	18	33
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than 12 months	$ 497	$ 1,908
Unrealized Losses, Less than 12 months	$ (3)	$ (29)
Count, Less than 12 months | security	1	4
Fair Value, 12 months or longer	$ 0	$ 0
Unrealized Losses, 12 months or longer	$ 0	$ 0
Count, 12 months or longer | security	0	0
Fair Value, Total	$ 497	$ 1,908
Unrealized Losses, Total	$ (3)	$ (29)
Count, Total | security	1	4