DEPOSITS (Deposit Liabilities) (Details) - USD ($)		Jun. 30, 2021	Dec. 31, 2020
Deposits:
Non-interest-bearing accounts		$ 6,090,063,000	$ 5,492,924,000
Interest-bearing checking		1,736,696,000	1,569,435,000
Regular savings accounts		2,646,302,000	2,398,482,000
Money market accounts		2,290,600,000	2,191,135,000
Total interest-bearing transaction and saving accounts		6,673,598,000	6,159,052,000
Certificates of deposit less than or equal to $250,000		678,155,000	718,256,000
Certificates of deposit greater than $250,000		194,892,000	197,064,000
Total certificates of deposit	[1]	873,047,000	915,320,000
Total deposits		13,636,708,000	12,567,296,000
Included in total deposits:
Public fund transaction and savings accounts		344,689,000	302,875,000
Public fund interest-bearing certificates		41,444,000	59,127,000
Total public deposits		386,133,000	362,002,000
Total brokered deposits		0	0
CD acquisition premium		$ 10,000	$ 58,000

[1] Certificates of deposit include $10,000 and $58,000 of acquisition discounts at June 30, 2021 and December 31, 2020, respectively.