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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, 2017 | Dec. 31, 2016
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Investment [Line Items] |  |
Held-to-Maturity, Amortized Cost | $ 594,776 | $ 562,434
Held-to-Maturity, Gross Unrealized Gains | 10,782 | 9,902
Held-to-Maturity, Gross Unrealized Losses | (2,263) | (3,405)
Held-to-Maturity, Fair Value | 603,295 | 568,931
States and Political Subdivisions [Member] |  |
Investment [Line Items] |  |
Held-to-Maturity, Amortized Cost | 429,088 | 394,875
Held-to-Maturity, Gross Unrealized Gains | 8,599 | 7,622
Held-to-Maturity, Gross Unrealized Losses | (1,895) | (3,014)
Held-to-Maturity, Fair Value | 435,792 | 399,483
Corporate Bonds [Member] |  |
Investment [Line Items] |  |
Held-to-Maturity, Amortized Cost | 140,553 | 143,858
Held-to-Maturity, Gross Unrealized Gains | 440 | 423
Held-to-Maturity, Gross Unrealized Losses | (256) | (265)
Held-to-Maturity, Fair Value | 140,737 | 144,016
Commercial Mortgage-Backed Securities [Member] |  |
Investment [Line Items] |  |
Held-to-Maturity, Amortized Cost |  | 70
Held-to-Maturity, Fair Value |  | 70
U.S. Agency-Based Mortgage-Backed Securities [Member] |  |
Investment [Line Items] |  |
Held-to-Maturity, Amortized Cost | 9,163 | 9,967
Held-to-Maturity, Gross Unrealized Gains | 861 | 948
Held-to-Maturity, Fair Value | 10,024 | 10,915
U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member] |  |
Investment [Line Items] |  |
Held-to-Maturity, Amortized Cost | 14,135 | 11,737
Held-to-Maturity, Gross Unrealized Gains | 723 | 746
Held-to-Maturity, Gross Unrealized Losses | (62) | (67)
Held-to-Maturity, Fair Value | 14,796 | 12,416
Asset-Backed Securities [Member] |  |
Investment [Line Items] |  |
Held-to-Maturity, Amortized Cost | 1,837 | 1,927
Held-to-Maturity, Gross Unrealized Gains | 159 | 163
Held-to-Maturity, Gross Unrealized Losses | (50) | (59)
Held-to-Maturity, Fair Value | $ 1,946 | $ 2,031

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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=73527792&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=65884222&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=73527792&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=73527792&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=73527792&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=73527792&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=73527792&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=73527792&loc=d3e27232-111563 + Details
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