SECURITIES (Amortized Cost, Gross Unrealized Gains and Losses, and Estimated Fair Values of Investment Securities) (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Total AFS securities	$ 2,088,851	$ 1,780,952
Gross Unrealized Gains	27,227	9,414
Gross Unrealized (Losses)	(7,016)	(15,545)
Estimated Fair Value	2,109,062	1,774,821
U.S. government and agency securities
Debt Securities, Available-for-sale [Line Items]
Total AFS securities	4,451
Gross Unrealized Gains	6
Gross Unrealized (Losses)	0
Estimated Fair Value	4,457
Obligations of states and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Total AFS securities	516,897	466,588
Gross Unrealized Gains	14,972	3,844
Gross Unrealized (Losses)	(81)	(1,941)
Estimated Fair Value	531,788	468,491
Corporate and other bonds
Debt Securities, Available-for-sale [Line Items]
Total AFS securities	177,910	167,561
Gross Unrealized Gains	1,784	1,118
Gross Unrealized (Losses)	(807)	(983)
Estimated Fair Value	178,887	167,696
Mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Total AFS securities	1,386,056	1,138,034
Gross Unrealized Gains	10,465	4,452
Gross Unrealized (Losses)	(6,128)	(12,621)
Estimated Fair Value	1,390,393	1,129,865
Other securities
Debt Securities, Available-for-sale [Line Items]
Total AFS securities	3,537	8,769
Gross Unrealized Gains	0	0
Gross Unrealized (Losses)	0	0
Estimated Fair Value	$ 3,537	$ 8,769