SECURITIES (Securities available for sale) (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 2,415,641	$ 2,492,620
Gross Unrealized Gains	12,632	4,662
Gross Unrealized Losses	(27,813)	(46,573)
Estimated Fair Value	2,400,460	2,450,709
Total debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,398,302	2,475,534
Gross Unrealized Gains	12,632	4,662
Gross Unrealized Losses	(27,813)	(46,573)
Estimated Fair Value	2,383,121	2,433,623
U.S. government corporations and agencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	26,873	32,075
Gross Unrealized Gains	10	3
Gross Unrealized Losses	(115)	(127)
Estimated Fair Value	26,768	31,951
Mortgage and asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,889,977	2,061,358
Gross Unrealized Gains	6,476	3,740
Gross Unrealized Losses	(25,055)	(38,400)
Estimated Fair Value	1,871,398	2,026,698
Obligations of states and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	481,452	382,101
Gross Unrealized Gains	6,146	919
Gross Unrealized Losses	(2,643)	(8,046)
Estimated Fair Value	484,955	374,974
Equity securities with a readily determinable fair value
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	17,339	17,086
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	$ 17,339	$ 17,086