INVESTMENT SECURITIES, Investment Securities in a Continuous Unrealized Loss Position (Details) $ in Thousands	Mar. 31, 2019 USD ($) Position	Dec. 31, 2018 USD ($) Position
Available-for-Sale Portfolio in Unrealized Loss Positions, Number of Positions [Abstract]
Less than 12 months | Position	13	169
12 months or longer | Position	289	384
Total | Position	302	553
Available-for-Sale Securities, in Unrealized Loss Position, Fair Value [Abstract]
Less than 12 months	$ 6,111	$ 576,527
12 months or longer	1,176,145	1,584,618
Total	1,182,256	2,161,145
Available-for-Sale Portfolio, Debt Maturities, Gross Unrealized Losses [Abstract]
Less than 12 months	8	1,156
12 months or longer	11,151	25,412
Total	$ 11,159	$ 26,568
Equity and Other Securities, Continuous Unrealized Loss Position, Number of Positions [Abstract]
Less than 12 months | Position	1	1
12 months or longer | Position	0	0
Total | Position	1	1
Equity and Other Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Less than 12 months	$ 98	$ 82
12 months or longer	0	0
Total	98	82
Equity and Other Securities, Continuous Unrealized Loss Position, Gross Unrealized Losses [Abstract]
Less than 12 months	2	19
12 months or longer	0	0
Total	$ 2	$ 19
U.S. Treasury and Agency Securities [Member]
Available-for-Sale Portfolio in Unrealized Loss Positions, Number of Positions [Abstract]
Less than 12 months | Position	0	7
12 months or longer | Position	48	64
Total | Position	48	71
Available-for-Sale Securities, in Unrealized Loss Position, Fair Value [Abstract]
Less than 12 months	$ 0	$ 473,082
12 months or longer	862,482	1,213,276
Total	862,482	1,686,358
Available-for-Sale Portfolio, Debt Maturities, Gross Unrealized Losses [Abstract]
Less than 12 months	0	682
12 months or longer	5,268	14,229
Total	$ 5,268	$ 14,911
Obligations of State and Political Subdivisions [Member]
Available-for-Sale Portfolio in Unrealized Loss Positions, Number of Positions [Abstract]
Less than 12 months | Position	3	118
12 months or longer | Position	20	97
Total | Position	23	215
Available-for-Sale Securities, in Unrealized Loss Position, Fair Value [Abstract]
Less than 12 months	$ 869	$ 55,671
12 months or longer	11,854	51,753
Total	12,723	107,424
Available-for-Sale Portfolio, Debt Maturities, Gross Unrealized Losses [Abstract]
Less than 12 months	2	216
12 months or longer	74	833
Total	$ 76	$ 1,049
Government Agency Mortgage-Backed Securities [Member]
Available-for-Sale Portfolio in Unrealized Loss Positions, Number of Positions [Abstract]
Less than 12 months | Position	9	43
12 months or longer | Position	181	181
Total | Position	190	224
Available-for-Sale Securities, in Unrealized Loss Position, Fair Value [Abstract]
Less than 12 months	$ 5,241	$ 47,708
12 months or longer	242,566	253,931
Total	247,807	301,639
Available-for-Sale Portfolio, Debt Maturities, Gross Unrealized Losses [Abstract]
Less than 12 months	6	258
12 months or longer	5,192	9,025
Total	$ 5,198	$ 9,283
Corporate Debt Securities [Member]
Available-for-Sale Portfolio in Unrealized Loss Positions, Number of Positions [Abstract]
Less than 12 months | Position	0	0
12 months or longer | Position	1	1
Total | Position	1	1
Available-for-Sale Securities, in Unrealized Loss Position, Fair Value [Abstract]
Less than 12 months	$ 0	$ 0
12 months or longer	2,553	2,546
Total	2,553	2,546
Available-for-Sale Portfolio, Debt Maturities, Gross Unrealized Losses [Abstract]
Less than 12 months	0	0
12 months or longer	19	42
Total	$ 19	$ 42
Government Agency Collateralized Mortgage Obligations [Member]
Available-for-Sale Portfolio in Unrealized Loss Positions, Number of Positions [Abstract]
Less than 12 months | Position	1	1
12 months or longer | Position	39	41
Total | Position	40	42
Available-for-Sale Securities, in Unrealized Loss Position, Fair Value [Abstract]
Less than 12 months	$ 1	$ 66
12 months or longer	56,690	63,112
Total	56,691	63,178
Available-for-Sale Portfolio, Debt Maturities, Gross Unrealized Losses [Abstract]
Less than 12 months	0	0
12 months or longer	598	1,283
Total	$ 598	$ 1,283
Equity Securities, at Fair Value [Member]
Equity and Other Securities, Continuous Unrealized Loss Position, Number of Positions [Abstract]
Less than 12 months | Position	1	1
12 months or longer | Position	0	0
Total | Position	1	1
Equity and Other Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Less than 12 months	$ 98	$ 82
12 months or longer	0	0
Total	98	82
Equity and Other Securities, Continuous Unrealized Loss Position, Gross Unrealized Losses [Abstract]
Less than 12 months	2	19
12 months or longer	0	0
Total	$ 2	$ 19