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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, 2018 | Dec. 31, 2017
--- | --- | ---
Investment [Line Items] |  |
Held-to-maturity, Amortized Cost, Totals | $ 619,400 | $ 629,668
Held-to-Maturity, Gross Unrealized Gains | 6,055 | 11,016
Held-to-Maturity, Gross Unrealized Losses | (4,519) | (1,375)
Held-to-Maturity, Fair Value | 620,936 | 639,309
States and Political Subdivisions [Member] |  |
Investment [Line Items] |  |
Held-to-maturity, Amortized Cost, Totals | 450,553 | 460,428
Held-to-Maturity, Gross Unrealized Gains | 5,041 | 9,628
Held-to-Maturity, Gross Unrealized Losses | (3,248) | (955)
Held-to-Maturity, Fair Value | 452,346 | 469,101
Corporate Bonds [Member] |  |
Investment [Line Items] |  |
Held-to-maturity, Amortized Cost, Totals | 99,368 | 100,024
Held-to-Maturity, Gross Unrealized Gains | 29 | 190
Held-to-Maturity, Gross Unrealized Losses | (650) | (167)
Held-to-Maturity, Fair Value | 98,747 | 100,047
U.S. Agency-Based Mortgage-Backed Securities [Member] |  |
Investment [Line Items] |  |
Held-to-maturity, Amortized Cost, Totals | 9,400 | 10,260
Held-to-Maturity, Gross Unrealized Gains | 503 | 625
Held-to-Maturity, Gross Unrealized Losses | (72) | (40)
Held-to-Maturity, Fair Value | 9,831 | 10,845
U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member] |  |
Investment [Line Items] |  |
Held-to-maturity, Amortized Cost, Totals | 58,857 | 57,657
Held-to-Maturity, Gross Unrealized Gains | 456 | 548
Held-to-Maturity, Gross Unrealized Losses | (542) | (198)
Held-to-Maturity, Fair Value | 58,771 | 58,007
Asset-Backed Securities [Member] |  |
Investment [Line Items] |  |
Held-to-maturity, Amortized Cost, Totals | 1,222 | 1,299
Held-to-Maturity, Gross Unrealized Gains | 26 | 25
Held-to-Maturity, Gross Unrealized Losses | (7) | (15)
Held-to-Maturity, Fair Value | $ 1,241 | $ 1,309

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- Definition Amount after other than temporary impairment (OTTI) accretion, of investments in debt securities classified as held-to-maturity. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -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 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 25 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=75030427&loc=d3e22054-111558 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (f) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27232-111563 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.1) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.6) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 + Details
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- Definition Amount of accumulated unrealized gain on investments in debt securities classified as held-to-maturity. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -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 investments in debt securities classified as held-to-maturity. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -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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- Definition Fair value of investments in debt securities classified as held-to-maturity. + References Reference 1: 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 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (f) -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 50 -Paragraph 5 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27232-111563 + Details
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