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INVESTMENT SECURITIES, Investment Securities in a Continuous Unrealized Loss Position (Details) $ in Thousands | Dec. 31, 2017 USD ($) Position | Dec. 31, 2016 USD ($) Position
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Available-for-Sale Securities in Unrealized Loss Positions, Number of Positions [Abstract] |  |
Less than 12 months | Position | 249 | 268
12 months or longer | Position | 56 | 17
Total | Position | 305 | 285
Available-for-Sale Securities, in Unrealized Loss Position, Fair Value [Abstract] |  |
Less than 12 months | $ 991,521 | $ 637,887
12 months or longer | 75,713 | 21,790
Total | 1,067,234 | 659,677
Available-for-Sale Securities, in Unrealized Loss Position, Gross Unrealized Losses [Abstract] |  |
Less than 12 months | 7,231 | 4,840
12 months or longer | 2,330 | 830
Total | $ 9,561 | $ 5,670
U.S. Treasury and Agency Securities [Member] |  |
Available-for-Sale Securities in Unrealized Loss Positions, Number of Positions [Abstract] |  |
Less than 12 months | Position | 44 | 13
12 months or longer | Position | 0 | 0
Total | Position | 44 | 13
Available-for-Sale Securities, in Unrealized Loss Position, Fair Value [Abstract] |  |
Less than 12 months | $ 699,709 | $ 449,242
12 months or longer | 0 | 0
Total | 699,709 | 449,242
Available-for-Sale Securities, in Unrealized Loss Position, Gross Unrealized Losses [Abstract] |  |
Less than 12 months | 4,838 | 2,118
12 months or longer | 0 | 0
Total | $ 4,838 | $ 2,118
Obligations of State and Political Subdivisions [Member] |  |
Available-for-Sale Securities in Unrealized Loss Positions, Number of Positions [Abstract] |  |
Less than 12 months | Position | 45 | 197
12 months or longer | Position | 0 | 0
Total | Position | 45 | 197
Available-for-Sale Securities, in Unrealized Loss Position, Fair Value [Abstract] |  |
Less than 12 months | $ 23,432 | $ 102,106
12 months or longer | 0 | 0
Total | 23,432 | 102,106
Available-for-Sale Securities, in Unrealized Loss Position, Gross Unrealized Losses [Abstract] |  |
Less than 12 months | 57 | 1,054
12 months or longer | 0 | 0
Total | $ 57 | $ 1,054
Government Agency Mortgage-Backed Securities [Member] |  |
Available-for-Sale Securities in Unrealized Loss Positions, Number of Positions [Abstract] |  |
Less than 12 months | Position | 120 | 57
12 months or longer | Position | 55 | 15
Total | Position | 175 | 72
Available-for-Sale Securities, in Unrealized Loss Position, Fair Value [Abstract] |  |
Less than 12 months | $ 185,716 | $ 83,862
12 months or longer | 75,712 | 21,788
Total | 261,428 | 105,650
Available-for-Sale Securities, in Unrealized Loss Position, Gross Unrealized Losses [Abstract] |  |
Less than 12 months | 1,433 | 1,637
12 months or longer | 2,330 | 830
Total | $ 3,763 | $ 2,467
Corporate Debt Securities [Member] |  |
Available-for-Sale Securities 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 | $ 2,623 | $ 2,677
12 months or longer | 0 | 0
Total | 2,623 | 2,677
Available-for-Sale Securities, in Unrealized Loss Position, Gross Unrealized Losses [Abstract] |  |
Less than 12 months | 25 | 31
12 months or longer | 0 | 0
Total | $ 25 | $ 31
Government Agency Collateralized Mortgage Obligations [Member] |  |
Available-for-Sale Securities in Unrealized Loss Positions, Number of Positions [Abstract] |  |
Less than 12 months | Position | 39 | 0
12 months or longer | Position | 1 | 2
Total | Position | 40 | 2
Available-for-Sale Securities, in Unrealized Loss Position, Fair Value [Abstract] |  |
Less than 12 months | $ 80,041 | $ 0
12 months or longer | 1 | 2
Total | 80,042 | 2
Available-for-Sale Securities, in Unrealized Loss Position, Gross Unrealized Losses [Abstract] |  |
Less than 12 months | 878 | 0
12 months or longer | 0 | 0
Total | $ 878 | $ 0

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- Definition Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for twelve months or longer. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 6 -Subparagraph (a)(2) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563 + Details
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- Definition Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale in a continuous loss position. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 6 -Subparagraph (a)(2) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563 + Details
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