DEPOSITS (Deposit Liabilities) (Details) - USD ($)		Mar. 31, 2020	Dec. 31, 2019
Deposits:
Non-interest-bearing accounts		$ 4,107,262,000	$ 3,945,000,000
Interest-bearing checking		1,331,860,000	1,280,003,000
Regular savings accounts		1,997,265,000	1,934,041,000
Money market accounts		1,846,844,000	1,769,194,000
Total interest-bearing transaction and saving accounts		5,175,969,000	4,983,238,000
Certificates of deposit less than or equal to $250,000		972,667,000	936,940,000
Certificates of deposit greater than $250,000		193,639,000	183,463,000
Total certificates of deposit	[1]	1,166,306,000	1,120,403,000
Total deposits		10,449,537,000	10,048,641,000
Included in total deposits:
Public fund transaction and savings accounts		246,312,000	244,418,000
Public fund interest-bearing certificates		48,232,000	35,184,000
Total public deposits		294,544,000	279,602,000
Total brokered deposits		250,977,000	202,884,000
CD acquisition premium		$ 206,000	$ 269,000

[1] Certificates of deposit include $206,000 and $269,000 of acquisition premiums at March 31, 2020 and December 31, 2019, respectively.