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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 | Mar. 31, 2019 | Dec. 31, 2018
--- | --- | ---
Investment [Line Items] |  |
Held-to-maturity, Amortized Cost, Totals | $ 604,056 | $ 613,878
Held-to-Maturity, Gross Unrealized Gains | 11,760 | 5,860
Held-to-Maturity, Gross Unrealized Losses | (599) | (2,966)
Held-to-Maturity, Fair Value | 615,217 | 616,772
States and Political Subdivisions [Member] |  |
Investment [Line Items] |  |
Held-to-maturity, Amortized Cost, Totals | 431,247 | 445,922
Held-to-Maturity, Gross Unrealized Gains | 10,468 | 5,109
Held-to-Maturity, Gross Unrealized Losses | (242) | (2,084)
Held-to-Maturity, Fair Value | 441,473 | 448,947
Corporate Bonds [Member] |  |
Investment [Line Items] |  |
Held-to-maturity, Amortized Cost, Totals | 102,774 | 91,762
Held-to-Maturity, Gross Unrealized Gains | 520 | 62
Held-to-Maturity, Gross Unrealized Losses | (155) | (455)
Held-to-Maturity, Fair Value | 103,139 | 91,369
U.S. Agency-Based Mortgage-Backed Securities [Member] |  |
Investment [Line Items] |  |
Held-to-maturity, Amortized Cost, Totals | 9,824 | 8,102
Held-to-Maturity, Gross Unrealized Gains | 396 | 327
Held-to-Maturity, Gross Unrealized Losses | (56) | (80)
Held-to-Maturity, Fair Value | 10,164 | 8,349
U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member] |  |
Investment [Line Items] |  |
Held-to-maturity, Amortized Cost, Totals | 59,210 | 67,042
Held-to-Maturity, Gross Unrealized Gains | 352 | 340
Held-to-Maturity, Gross Unrealized Losses | (140) | (339)
Held-to-Maturity, Fair Value | 59,422 | 67,043
Asset-Backed Securities [Member] |  |
Investment [Line Items] |  |
Held-to-maturity, Amortized Cost, Totals | 1,001 | 1,050
Held-to-Maturity, Gross Unrealized Gains | 24 | 22
Held-to-Maturity, Gross Unrealized Losses | (6) | (8)
Held-to-Maturity, Fair Value | $ 1,019 | $ 1,064

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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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