INVESTMENT SECURITIES, Investment Securities in a Continuous Unrealized Loss Position (Details) $ in Thousands	Jun. 30, 2018 USD ($) Position	Dec. 31, 2017 USD ($) Position
Available-for-Sale Portfolio in Unrealized Loss Positions, Number of Positions [Abstract]
Less than 12 months | Position	469	249
12 months or longer | Position	89	56
Total | Position	558	305
Available-for-Sale Securities, in Unrealized Loss Position, Fair Value [Abstract]
Less than 12 months	$ 2,312,116	$ 991,521
12 months or longer	112,099	75,713
Total	2,424,215	1,067,234
Available-for-Sale Portfolio, Debt Maturities, Gross Unrealized Losses [Abstract]
Less than 12 months	37,815	7,231
12 months or longer	5,208	2,330
Total	$ 43,023	$ 9,561
U.S. Treasury and Agency Securities [Member]
Available-for-Sale Portfolio in Unrealized Loss Positions, Number of Positions [Abstract]
Less than 12 months | Position	77	44
12 months or longer | Position	3	0
Total | Position	80	44
Available-for-Sale Securities, in Unrealized Loss Position, Fair Value [Abstract]
Less than 12 months	$ 1,924,896	$ 699,709
12 months or longer	14,945	0
Total	1,939,841	699,709
Available-for-Sale Portfolio, Debt Maturities, Gross Unrealized Losses [Abstract]
Less than 12 months	28,567	4,838
12 months or longer	64	0
Total	$ 28,631	$ 4,838
Obligations of State and Political Subdivisions [Member]
Available-for-Sale Portfolio in Unrealized Loss Positions, Number of Positions [Abstract]
Less than 12 months | Position	224	45
12 months or longer | Position	0	0
Total | Position	224	45
Available-for-Sale Securities, in Unrealized Loss Position, Fair Value [Abstract]
Less than 12 months	$ 111,946	$ 23,432
12 months or longer	0	0
Total	111,946	23,432
Available-for-Sale Portfolio, Debt Maturities, Gross Unrealized Losses [Abstract]
Less than 12 months	1,440	57
12 months or longer	0	0
Total	$ 1,440	$ 57
Government Agency Mortgage-Backed Securities [Member]
Available-for-Sale Portfolio in Unrealized Loss Positions, Number of Positions [Abstract]
Less than 12 months | Position	135	120
12 months or longer | Position	77	55
Total | Position	212	175
Available-for-Sale Securities, in Unrealized Loss Position, Fair Value [Abstract]
Less than 12 months	$ 213,903	$ 185,716
12 months or longer	85,735	75,712
Total	299,638	261,428
Available-for-Sale Portfolio, Debt Maturities, Gross Unrealized Losses [Abstract]
Less than 12 months	5,885	1,433
12 months or longer	4,861	2,330
Total	$ 10,746	$ 3,763
Corporate Debt Securities [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	0	0
Total | Position	1	1
Available-for-Sale Securities, in Unrealized Loss Position, Fair Value [Abstract]
Less than 12 months	$ 2,563	$ 2,623
12 months or longer	0	0
Total	2,563	2,623
Available-for-Sale Portfolio, Debt Maturities, Gross Unrealized Losses [Abstract]
Less than 12 months	55	25
12 months or longer	0	0
Total	$ 55	$ 25
Government Agency Collateralized Mortgage Obligations [Member]
Available-for-Sale Portfolio in Unrealized Loss Positions, Number of Positions [Abstract]
Less than 12 months | Position	32	39
12 months or longer | Position	9	1
Total | Position	41	40
Available-for-Sale Securities, in Unrealized Loss Position, Fair Value [Abstract]
Less than 12 months	$ 58,808	$ 80,041
12 months or longer	11,419	1
Total	70,227	80,042
Available-for-Sale Portfolio, Debt Maturities, Gross Unrealized Losses [Abstract]
Less than 12 months	1,868	878
12 months or longer	283	0
Total	$ 2,151	$ 878