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Investments - Gross Unrealized Gains and Losses and Amortized Cost and Fair Value of Investments Classified as Held-to-Maturity (Detail) - USD ($) $ in Thousands | Sep. 30, 2018 | Dec. 31, 2017
--- | --- | ---
Investment [Line Items] |  |
Held-to-maturity, Amortized Cost, Totals | $ 602,830 | $ 629,668
Held-to-Maturity, Gross Unrealized Gains | 3,770 | 11,016
Held-to-Maturity, Gross Unrealized Losses | (6,715) | (1,375)
Held-to-Maturity, Fair Value | 599,885 | 639,309
States and Political Subdivisions [Member] |  |
Investment [Line Items] |  |
Held-to-maturity, Amortized Cost, Totals | 430,408 | 460,428
Held-to-Maturity, Gross Unrealized Gains | 3,053 | 9,628
Held-to-Maturity, Gross Unrealized Losses | (4,773) | (955)
Held-to-Maturity, Fair Value | 428,688 | 469,101
Corporate Bonds [Member] |  |
Investment [Line Items] |  |
Held-to-maturity, Amortized Cost, Totals | 95,731 | 100,024
Held-to-Maturity, Gross Unrealized Gains | 9 | 190
Held-to-Maturity, Gross Unrealized Losses | (879) | (167)
Held-to-Maturity, Fair Value | 94,861 | 100,047
U.S. Agency-Based Mortgage-Backed Securities [Member] |  |
Investment [Line Items] |  |
Held-to-maturity, Amortized Cost, Totals | 8,535 | 10,260
Held-to-Maturity, Gross Unrealized Gains | 359 | 625
Held-to-Maturity, Gross Unrealized Losses | (87) | (40)
Held-to-Maturity, Fair Value | 8,807 | 10,845
U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member] |  |
Investment [Line Items] |  |
Held-to-maturity, Amortized Cost, Totals | 67,052 | 57,657
Held-to-Maturity, Gross Unrealized Gains | 325 | 548
Held-to-Maturity, Gross Unrealized Losses | (966) | (198)
Held-to-Maturity, Fair Value | 66,411 | 58,007
Asset-Backed Securities [Member] |  |
Investment [Line Items] |  |
Held-to-maturity, Amortized Cost, Totals | 1,104 | 1,299
Held-to-Maturity, Gross Unrealized Gains | 24 | 25
Held-to-Maturity, Gross Unrealized Losses | (10) | (15)
Held-to-Maturity, Fair Value | $ 1,118 | $ 1,309

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- Definition Amount of investment in debt security measured at amortized cost (held-to-maturity). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=109979856&loc=d3e26610-111562 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27232-111563 + Details
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- Definition Amount of accumulated unrecognized gain on investment in debt security measured at amortized cost (held-to-maturity). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -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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- Definition Amount of accumulated unrealized loss on investment in debt security measured at amortized cost (held-to-maturity). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27232-111563 + Details
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