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INVESTMENT SECURITIES, Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Details) - USD ($) $ in Thousands | Mar. 31, 2018 | Dec. 31, 2017
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Available-for-Sale, Debt Maturities, Amortized Cost [Abstract] |  |
Due in one year or less | $ 53,102 |
Due after one through five years | 1,557,644 |
Due after five years through ten years | 789,557 |
Due after ten years | 149,642 |
Subtotal | 2,549,945 |
Amortized cost | 3,004,915 |
Available-for-Sale, Debt Maturities, Fair Value [Abstract] |  |
Due in one year or less | 53,115 |
Due after one through five years | 1,546,876 |
Due after five years through ten years | 788,557 |
Due after ten years | 152,253 |
Subtotal | 2,540,801 |
Fair value | 2,986,769 |
Government Agency Mortgage-Backed Securities [Member] |  |
Available-for-Sale, Debt Maturities, Amortized Cost [Abstract] |  |
Without single maturity date | 371,857 |
Amortized cost | 371,857 | $ 358,180
Available-for-Sale, Debt Maturities, Fair Value [Abstract] |  |
Without single maturity date | 364,539 |
Fair value | 364,539 | 357,538
Government Agency Collateralized Mortgage Obligations [Member] |  |
Available-for-Sale, Debt Maturities, Amortized Cost [Abstract] |  |
Without single maturity date | 83,113 |
Amortized cost | 83,113 | 88,097
Available-for-Sale, Debt Maturities, Fair Value [Abstract] |  |
Without single maturity date | 81,429 |
Fair value | 81,429 | 87,374
U.S. Treasury Securities [Member] |  |
Available-for-Sale, Debt Maturities, Amortized Cost [Abstract] |  |
Amortized cost | 2,044,795 | 2,043,023
Available-for-Sale, Debt Maturities, Fair Value [Abstract] |  |
Fair value | 2,027,914 | $ 2,054,071
Securities pledged as collateral for securities sold under agreement to repurchase | $ 279,700 |

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- Definition This item represents the cost of debt securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (aa) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.2) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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- Definition Amount after adjustments of available-for-sale debt securities at cost, maturing in the next rolling twelve months following the latest balance sheet. Adjustments include, but are not limited to, accretion, amortization, collection of cash, previous other-than-temporary impairments (OTTI) recognized in earnings less cumulative-effect adjustments and fair value hedge accounting adjustments. For interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date. + References No definition available. + Details
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- Definition Amount after adjustments of available-for-sale debt securities at cost, maturing after the tenth rolling twelve months following the latest balance sheet. Adjustments include, but are not limited to, accretion, amortization, collection of cash, previous other-than-temporary impairments (OTTI) recognized in earnings less cumulative-effect adjustments, and fair value hedge accounting adjustments. For interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date. + References No definition available. + Details
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- Definition Amount after adjustments of available-for-sale debt securities at cost, maturing in the sixth through tenth rolling twelve months following the latest balance sheet. Adjustments include, but are not limited to, accretion, amortization, collection of cash, previous other-than-temporary impairments (OTTI) recognized in earnings less cumulative-effect adjustments, and fair value hedge accounting adjustments. For interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date. + References No definition available. + Details
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- Definition Amount after adjustments of available-for-sale debt securities at cost, maturing in the second through fifth rolling twelve months following the latest balance sheet. Adjustments include, but are not limited to, accretion, amortization, collection of cash, previous other-than-temporary impairments (OTTI) recognized in earnings less cumulative-effect adjustments, and fair value hedge accounting adjustments. For interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date. + References No definition available. + Details
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- Definition Amount of available-for-sale debt securities at cost, net of adjustments, that do not have a single maturity date and which the entity has decided to disclose separately rather than allocating the cost over several maturity groupings. Adjustments include, but are not limited to, accretion, amortization, collection of cash, previous other-than-temporary impairments (OTTI) recognized in earnings (less any cumulative-effect adjustments, as defined) and fair value hedge accounting adjustments. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27198-111563 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.2) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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