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INVESTMENT SECURITIES, Investment Securities in a Continuous Unrealized Loss Position (Details) $ in Thousands	Sep. 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	484	249
12 months or longer | Position	214	56
Total | Position	698	305
Available-for-Sale Securities, in Unrealized Loss Position, Fair Value [Abstract]
Less than 12 months	$ 1,942,763	$ 991,521
12 months or longer	611,395	75,713
Total	2,554,158	1,067,234
Available-for-Sale Portfolio, Debt Maturities, Gross Unrealized Losses [Abstract]
Less than 12 months	37,226	7,231
12 months or longer	22,386	2,330
Total	$ 59,612	$ 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	50	44
12 months or longer | Position	32	0
Total | Position	82	44
Available-for-Sale Securities, in Unrealized Loss Position, Fair Value [Abstract]
Less than 12 months	$ 1,633,859	$ 699,709
12 months or longer	364,236	0
Total	1,998,095	699,709
Available-for-Sale Portfolio, Debt Maturities, Gross Unrealized Losses [Abstract]
Less than 12 months	30,625	4,838
12 months or longer	10,366	0
Total	$ 40,991	$ 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	333	45
12 months or longer | Position	9	0
Total | Position	342	45
Available-for-Sale Securities, in Unrealized Loss Position, Fair Value [Abstract]
Less than 12 months	$ 171,781	$ 23,432
12 months or longer	5,913	0
Total	177,694	23,432
Available-for-Sale Portfolio, Debt Maturities, Gross Unrealized Losses [Abstract]
Less than 12 months	2,619	57
12 months or longer	314	0
Total	$ 2,933	$ 57
Government Agency Mortgage-Backed Securities [Member]
Available-for-Sale Portfolio in Unrealized Loss Positions, Number of Positions [Abstract]
Less than 12 months | Position	89	120
12 months or longer | Position	136	55
Total | Position	225	175
Available-for-Sale Securities, in Unrealized Loss Position, Fair Value [Abstract]
Less than 12 months	$ 128,752	$ 185,716
12 months or longer	178,871	75,712
Total	307,623	261,428
Available-for-Sale Portfolio, Debt Maturities, Gross Unrealized Losses [Abstract]
Less than 12 months	3,783	1,433
12 months or longer	9,421	2,330
Total	$ 13,204	$ 3,763
Corporate Debt Securities [Member]
Available-for-Sale Portfolio in Unrealized Loss Positions, Number of Positions [Abstract]
Less than 12 months | Position	0	1
12 months or longer | Position	1	0
Total | Position	1	1
Available-for-Sale Securities, in Unrealized Loss Position, Fair Value [Abstract]
Less than 12 months	$ 0	$ 2,623
12 months or longer	2,550	0
Total	2,550	2,623
Available-for-Sale Portfolio, Debt Maturities, Gross Unrealized Losses [Abstract]
Less than 12 months	0	25
12 months or longer	53	0
Total	$ 53	$ 25
Government Agency Collateralized Mortgage Obligations [Member]
Available-for-Sale Portfolio in Unrealized Loss Positions, Number of Positions [Abstract]
Less than 12 months | Position	12	39
12 months or longer | Position	36	1
Total | Position	48	40
Available-for-Sale Securities, in Unrealized Loss Position, Fair Value [Abstract]
Less than 12 months	$ 8,371	$ 80,041
12 months or longer	59,825	1
Total	68,196	80,042
Available-for-Sale Portfolio, Debt Maturities, Gross Unrealized Losses [Abstract]
Less than 12 months	199	878
12 months or longer	2,232	0
Total	$ 2,431	$ 878

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for more than 12 months.

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