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Securities - Summary of Amortized Cost and Estimated Fair Values of Securities Available for Sale (Detail) - USD ($) $ in Thousands | Dec. 31, 2017 | Dec. 31, 2016
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Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | $ 470,825 | $ 465,315
Gross Unrealized Gains | 911 | 2,324
Gross Unrealized Losses | (4,033) | (2,171)
Estimated Fair Value | 467,703 | 465,468
U.S. Treasuries [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 314,905 | 268,763
Gross Unrealized Gains |  | 700
Gross Unrealized Losses | (2,103) | (920)
Estimated Fair Value | 312,802 | 268,543
U.S. Federal Agencies [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 89,098 | 129,674
Gross Unrealized Gains | 82 | 373
Gross Unrealized Losses | (329) | (405)
Estimated Fair Value | 88,851 | 129,642
Mortgage Backed Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 18,358 | 19,949
Gross Unrealized Gains | 204 | 290
Gross Unrealized Losses | (586) | (567)
Estimated Fair Value | 17,976 | 19,672
States and Political Subdivisions [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 41,937 | 40,335
Gross Unrealized Gains | 554 | 836
Gross Unrealized Losses | (121) | (129)
Estimated Fair Value | 42,370 | 41,042
Other Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 6,527 | 6,594
Gross Unrealized Gains | 71 | 125
Gross Unrealized Losses | (894) | (150)
Estimated Fair Value | $ 5,704 | $ 6,569

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- Definition Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 25 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=75030427&loc=d3e22054-111558 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (aa) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27232-111563 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=82896770&loc=d3e26610-111562 + Details
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- Definition Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity 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 and equity 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 and equity 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 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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