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Investment Securities - Amortized Cost and Approximate Fair Values of Investment Securities (Details) - USD ($) $ in Thousands | Mar. 31, 2018 | Dec. 31, 2017
--- | --- | ---
Available-for-Sale: |  |
Amortized Cost | $ 2,325,000 | $ 2,229,300
Gross Unrealized Gains | 2,000 | 3,900
Gross Unrealized Losses | (46,300) | (24,500)
Estimated Fair Value | 2,280,700 | 2,208,700
Held-to-Maturity: |  |
Amortized Cost | 462,300 | 484,500
Gross Unrealized Gains | 9,600 | 11,500
Gross Unrealized Losses | (16,400) | (12,700)
Estimated Fair Value | 455,500 | 483,300
U.S. Treasury notes |  |
Available-for-Sale: |  |
Amortized Cost | 3,200 | 3,200
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | 0 | 0
Estimated Fair Value | 3,200 | 3,200
Obligations of U.S. government agencies |  |
Available-for-Sale: |  |
Amortized Cost | 562,800 | 569,500
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | (13,000) | (8,000)
Estimated Fair Value | 549,800 | 561,500
Held-to-Maturity: |  |
Amortized Cost | 19,800 | 19,800
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | (300) | (200)
Estimated Fair Value | 19,500 | 19,600
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations |  |
Available-for-Sale: |  |
Amortized Cost | 1,578,200 | 1,474,100
Gross Unrealized Gains | 1,900 | 3,800
Gross Unrealized Losses | (30,800) | (15,400)
Estimated Fair Value | 1,549,300 | 1,462,500
Held-to-Maturity: |  |
Amortized Cost | 217,600 | 230,500
Gross Unrealized Gains | 7,700 | 8,800
Gross Unrealized Losses | (14,100) | (11,600)
Estimated Fair Value | 211,200 | 227,700
Private mortgage-backed securities |  |
Available-for-Sale: |  |
Amortized Cost | 84,700 | 91,500
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | (1,500) | (800)
Estimated Fair Value | 83,200 | 90,700
State, county and municipal securities |  |
Held-to-Maturity: |  |
Amortized Cost | 168,300 | 172,400
Gross Unrealized Gains | 1,800 | 2,600
Gross Unrealized Losses | (1,300) | (600)
Estimated Fair Value | 168,800 | 174,400
Corporate securities |  |
Available-for-Sale: |  |
Amortized Cost | 93,100 | 88,000
Gross Unrealized Gains | 100 | 100
Gross Unrealized Losses | (1,000) | (300)
Estimated Fair Value | 92,200 | 87,800
Held-to-Maturity: |  |
Amortized Cost | 56,500 | 61,600
Gross Unrealized Gains | 100 | 100
Gross Unrealized Losses | (700) | (300)
Estimated Fair Value | 55,900 | 61,400
Other investments |  |
Available-for-Sale: |  |
Amortized Cost | 3,000 | 3,000
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | 0 | 0
Estimated Fair Value | 3,000 | 3,000
Other Investments |  |
Held-to-Maturity: |  |
Amortized Cost | 100 | 200
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | 0 | 0
Estimated Fair Value | $ 100 | $ 200

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- Definition Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt securities classified as available-for-sale. + 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 (b) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563 + Details
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- Definition Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt securities classified as available-for-sale. + 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 (c) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563 + Details
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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 of accumulated unrealized gain on investments in debt securities classified as held-to-maturity. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27232-111563 + Details
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