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Securities - Gross Unrealized Losses, Aggregated by Investment Category and Length of Time (Detail) - USD ($) $ in Thousands | Dec. 31, 2017 | Dec. 31, 2016
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Schedule of Available-for-sale Securities [Line Items] |  |
Gross Unrealized Losses, Less Than Twelve Months | $ (625) | $ (3,803)
Fair Value, Less Than Twelve Months | 66,400 | 167,230
Gross Unrealized Losses, Twelve Months and Over | (2,643) | (7)
Fair Value, Twelve Months and Over | 106,840 | 526
U.S. Treasury [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Gross Unrealized Losses, Less Than Twelve Months | (36) | (146)
Fair Value, Less Than Twelve Months | 6,924 | 15,745
Gross Unrealized Losses, Twelve Months and Over | (205) |
Fair Value, Twelve Months and Over | 14,054 |
U.S. Government Agencies [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Gross Unrealized Losses, Less Than Twelve Months | (314) | (1,795)
Fair Value, Less Than Twelve Months | 27,328 | 77,471
Gross Unrealized Losses, Twelve Months and Over | (1,418) |
Fair Value, Twelve Months and Over | 53,139 |
Mortgage-backed Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Gross Unrealized Losses, Less Than Twelve Months | (70) | (879)
Fair Value, Less Than Twelve Months | 7,149 | 36,474
Gross Unrealized Losses, Twelve Months and Over | (720) |
Fair Value, Twelve Months and Over | 28,080 |
State and Local Governments [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Gross Unrealized Losses, Less Than Twelve Months | (205) | (983)
Fair Value, Less Than Twelve Months | 24,999 | 37,540
Gross Unrealized Losses, Twelve Months and Over | (300) | (7)
Fair Value, Twelve Months and Over | $ 11,567 | $ 526

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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 that have been in a continuous loss position for less than twelve months. + 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 This item represents the aggregate fair value of investments in debt and equity securities categorized neither as held-to-maturity nor trading securities that have been in a continuous unrealized loss position for less than twelve months. + 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)(1) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563 + Details
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