INVESTMENT SECURITIES (Investment Securities at an Unrealized Loss Position) (Details) $ in Thousands	Dec. 31, 2017 USD ($) security	Dec. 31, 2016 USD ($) security
Investments, Debt and Equity Securities [Abstract]
Number of investment securities in an unrealized loss position | security	223	242
Total temporary impaired securities
Less than 12 months, Fair Value	$ 559,687	$ 930,710
Less than 12 months, Unrealized Losses	(4,662)	(15,836)
12 months or longer, Fair Value	360,929	3,978
12 months or longer, Unrealized Losses	(9,129)	(223)
Total, Fair Value	920,616	934,688
Total, Unrealized Losses	(13,791)	(16,059)
States and political subdivisions
Total temporary impaired securities
Less than 12 months, Fair Value	53,811	85,288
Less than 12 months, Unrealized Losses	(305)	(1,797)
12 months or longer, Fair Value	15,403	0
12 months or longer, Unrealized Losses	(414)	0
Total, Fair Value	69,214	85,288
Total, Unrealized Losses	(719)	(1,797)
Corporate securities
Total temporary impaired securities
Less than 12 months, Fair Value	0	20,357
Less than 12 months, Unrealized Losses	0	(181)
12 months or longer, Fair Value	5,307	0
12 months or longer, Unrealized Losses	(76)	0
Total, Fair Value	5,307	20,357
Total, Unrealized Losses	(76)	(181)
U.S. Treasury obligations and direct obligations of U.S Government agencies
Total temporary impaired securities
Less than 12 months, Fair Value	10,740
Less than 12 months, Unrealized Losses	(69)
12 months or longer, Fair Value	0
12 months or longer, Unrealized Losses	0
Total, Fair Value	10,740
Total, Unrealized Losses	(69)
Residential-U.S. Government sponsored entities
Total temporary impaired securities
Less than 12 months, Fair Value	335,883	648,923
Less than 12 months, Unrealized Losses	(3,372)	(11,766)
12 months or longer, Fair Value	340,219	3,978
12 months or longer, Unrealized Losses	(8,639)	(223)
Total, Fair Value	676,102	652,901
Total, Unrealized Losses	(12,011)	(11,989)
Residential - Non-government sponsored entities
Total temporary impaired securities
Less than 12 months, Fair Value		30,596
Less than 12 months, Unrealized Losses		(761)
12 months or longer, Fair Value		0
12 months or longer, Unrealized Losses		0
Total, Fair Value		30,596
Total, Unrealized Losses		(761)
Commercial-U.S. Government sponsored entities
Total temporary impaired securities
Less than 12 months, Fair Value	130,763	92,666
Less than 12 months, Unrealized Losses	(723)	(920)
12 months or longer, Fair Value	0	0
12 months or longer, Unrealized Losses	0	0
Total, Fair Value	130,763	92,666
Total, Unrealized Losses	(723)	(920)
Commercial-Non-government sponsored entities
Total temporary impaired securities
Less than 12 months, Fair Value	28,490	52,880
Less than 12 months, Unrealized Losses	(193)	(411)
12 months or longer, Fair Value	0	0
12 months or longer, Unrealized Losses	0	0
Total, Fair Value	28,490	52,880
Total, Unrealized Losses	$ (193)	$ (411)