SECURITIES (Debt securities available for sale) (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Fair Value
Less than 12 months	$ 272,805
12 months or longer	1,063,232
Total	1,336,037
Less than 12 months		$ 793,718
12 months or longer		1,067,809
Total		1,861,527
Unrealized Losses
Less than 12 months	(2,268)
12 months or longer	(25,545)
Total	(27,813)
Less than 12 months		(9,533)
12 months or longer		(37,040)
Total		(46,573)
U.S. government corporations and agencies
Fair Value
Less than 12 months	18,742
12 months or longer	4,541
Total	23,283
Less than 12 months		24,902
12 months or longer		4,577
Total		29,479
Unrealized Losses
Less than 12 months	(95)
12 months or longer	(20)
Total	(115)
Less than 12 months		(83)
12 months or longer		(44)
Total		(127)
Mortgage and asset-backed securities
Fair Value
Less than 12 months	227,902
12 months or longer	926,176
Total	1,154,078
Less than 12 months		733,826
12 months or longer		805,089
Total		1,538,915
Unrealized Losses
Less than 12 months	(2,075)
12 months or longer	(22,980)
Total	(25,055)
Less than 12 months		(9,060)
12 months or longer		(29,340)
Total		(38,400)
Obligations of states and political subdivisions
Fair Value
Less than 12 months	26,161
12 months or longer	132,515
Total	158,676
Less than 12 months		34,990
12 months or longer		258,143
Total		293,133
Unrealized Losses
Less than 12 months	(98)
12 months or longer	(2,545)
Total	$ (2,643)
Less than 12 months		(390)
12 months or longer		(7,656)
Total		$ (8,046)