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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 | Dec. 31, 2018 | Dec. 31, 2017
--- | --- | ---
Investment [Line Items] |  |
Held-to-maturity, Amortized Cost, Totals | $ 613,878 | $ 629,668
Held-to-Maturity, Gross Unrealized Gains | 5,860 | 11,016
Held-to-Maturity, Gross Unrealized Losses | (2,966) | (1,375)
Held-to-Maturity, Fair Value | 616,772 | 639,309
States and Political Subdivisions [Member] |  |
Investment [Line Items] |  |
Held-to-maturity, Amortized Cost, Totals | 445,922 | 460,428
Held-to-Maturity, Gross Unrealized Gains | 5,109 | 9,628
Held-to-Maturity, Gross Unrealized Losses | (2,084) | (955)
Held-to-Maturity, Fair Value | 448,947 | 469,101
Corporate Bonds [Member] |  |
Investment [Line Items] |  |
Held-to-maturity, Amortized Cost, Totals | 91,762 | 100,024
Held-to-Maturity, Gross Unrealized Gains | 62 | 190
Held-to-Maturity, Gross Unrealized Losses | (455) | (167)
Held-to-Maturity, Fair Value | 91,369 | 100,047
U.S. Agency-Based Mortgage-Backed Securities [Member] |  |
Investment [Line Items] |  |
Held-to-maturity, Amortized Cost, Totals | 8,102 | 10,260
Held-to-Maturity, Gross Unrealized Gains | 327 | 625
Held-to-Maturity, Gross Unrealized Losses | (80) | (40)
Held-to-Maturity, Fair Value | 8,349 | 10,845
U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member] |  |
Investment [Line Items] |  |
Held-to-maturity, Amortized Cost, Totals | 67,042 | 57,657
Held-to-Maturity, Gross Unrealized Gains | 340 | 548
Held-to-Maturity, Gross Unrealized Losses | (339) | (198)
Held-to-Maturity, Fair Value | 67,043 | 58,007
Asset-Backed Securities [Member] |  |
Investment [Line Items] |  |
Held-to-maturity, Amortized Cost, Totals | 1,050 | 1,299
Held-to-Maturity, Gross Unrealized Gains | 22 | 25
Held-to-Maturity, Gross Unrealized Losses | (8) | (15)
Held-to-Maturity, Fair Value | $ 1,064 | $ 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 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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