SECURITIES (Debt securities available for sale) (Details) $ in Thousands	Mar. 31, 2021 USD ($) bankLocation	Dec. 31, 2020 USD ($) bankLocation
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than 12 months	$ 2,626,514	$ 1,672,530
Unrealized Losses, Less than 12 months	$ (76,841)	$ (18,373)
Count, Less than 12 months \| bankLocation	262	127
Fair Value, 12 months or longer	$ 289,669	$ 508,436
Unrealized Losses, 12 months or longer	$ (2,907)	$ (5,743)
Count, 12 months or longer \| bankLocation	98	103
Fair Value, Total	$ 2,916,183	$ 2,180,966
Unrealized Losses, Total	$ (79,748)	$ (24,116)
Count, Total \| bankLocation	360	230
U.S. treasuries
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than 12 months	$ 0	$ 0
Unrealized Losses, Less than 12 months	$ 0	$ 0
Count, Less than 12 months \| bankLocation	0	0
Fair Value, 12 months or longer	$ 0	$ 0
Unrealized Losses, 12 months or longer	$ 0	$ 0
Count, 12 months or longer \| bankLocation	0	0
Fair Value, Total	$ 0	$ 0
Unrealized Losses, Total	$ 0	$ 0
Count, Total \| bankLocation	0	0
U.S. agencies
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than 12 months	$ 23,581	$ 2,981
Unrealized Losses, Less than 12 months	$ (997)	$ (8)
Count, Less than 12 months \| bankLocation	2	5
Fair Value, 12 months or longer	$ 95,757	$ 99,922
Unrealized Losses, 12 months or longer	$ (738)	$ (918)
Count, 12 months or longer \| bankLocation	73	72
Fair Value, Total	$ 119,338	$ 102,903
Unrealized Losses, Total	$ (1,735)	$ (926)
Count, Total \| bankLocation	75	77
Obligations of states and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than 12 months	$ 1,000,647	$ 346,598
Unrealized Losses, Less than 12 months	$ (40,997)	$ (2,959)
Count, Less than 12 months \| bankLocation	169	49
Fair Value, 12 months or longer	$ 13,126	$ 0
Unrealized Losses, 12 months or longer	$ (112)	$ 0
Count, 12 months or longer \| bankLocation	2	0
Fair Value, Total	$ 1,013,773	$ 346,598
Unrealized Losses, Total	$ (41,109)	$ (2,959)
Count, Total \| bankLocation	171	49
Mortgage-backed securities - agency
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than 12 months	$ 774,060	$ 653,793
Unrealized Losses, Less than 12 months	$ (28,762)	$ (12,342)
Count, Less than 12 months \| bankLocation	51	35
Fair Value, 12 months or longer	$ 27,063	$ 31,012
Unrealized Losses, 12 months or longer	$ (406)	$ (381)
Count, 12 months or longer \| bankLocation	2	3
Fair Value, Total	$ 801,123	$ 684,805
Unrealized Losses, Total	$ (29,168)	$ (12,723)
Count, Total \| bankLocation	53	38
Mortgage-backed securities - non-agency
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than 12 months	$ 568,107	$ 378,843
Unrealized Losses, Less than 12 months	$ (1,801)	$ (1,639)
Count, Less than 12 months \| bankLocation	18	17
Fair Value, 12 months or longer	$ 30,172	$ 1,622
Unrealized Losses, 12 months or longer	$ (308)	$ (1)
Count, 12 months or longer \| bankLocation	5	1
Fair Value, Total	$ 598,279	$ 380,465
Unrealized Losses, Total	$ (2,109)	$ (1,640)
Count, Total \| bankLocation	23	18
Commercial mortgage-backed securities - agency
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than 12 months	$ 72,262	$ 46,541
Unrealized Losses, Less than 12 months	$ (3,717)	$ (738)
Count, Less than 12 months \| bankLocation	11	6
Fair Value, 12 months or longer	$ 0	$ 0
Unrealized Losses, 12 months or longer	$ 0	$ 0
Count, 12 months or longer \| bankLocation	0	0
Fair Value, Total	$ 72,262	$ 46,541
Unrealized Losses, Total	$ (3,717)	$ (738)
Count, Total \| bankLocation	11	6
Commercial mortgage-backed securities - non-agency
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than 12 months	$ 57,789	$ 100,042
Unrealized Losses, Less than 12 months	$ (6)	$ (15)
Count, Less than 12 months \| bankLocation	1	2
Fair Value, 12 months or longer	$ 34,659	$ 35,428
Unrealized Losses, 12 months or longer	$ (236)	$ (676)
Count, 12 months or longer \| bankLocation	4	3
Fair Value, Total	$ 92,448	$ 135,470
Unrealized Losses, Total	$ (242)	$ (691)
Count, Total \| bankLocation	5	5
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than 12 months	$ 128,354	$ 141,824
Unrealized Losses, Less than 12 months	$ (533)	$ (643)
Count, Less than 12 months \| bankLocation	6	9
Fair Value, 12 months or longer	$ 88,892	$ 340,452
Unrealized Losses, 12 months or longer	$ (1,107)	$ (3,767)
Count, 12 months or longer \| bankLocation	12	24
Fair Value, Total	$ 217,246	$ 482,276
Unrealized Losses, Total	$ (1,640)	$ (4,410)
Count, Total \| bankLocation	18	33
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than 12 months	$ 1,714	$ 1,908
Unrealized Losses, Less than 12 months	$ (28)	$ (29)
Count, Less than 12 months \| bankLocation	4	4
Fair Value, 12 months or longer	$ 0	$ 0
Unrealized Losses, 12 months or longer	$ 0	$ 0
Count, 12 months or longer \| bankLocation	0	0
Fair Value, Total	$ 1,714	$ 1,908
Unrealized Losses, Total	$ (28)	$ (29)
Count, Total \| bankLocation	4	4

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for more than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

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