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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 | Jun. 30, 2017 | Dec. 31, 2016
--- | --- | ---
Investment [Line Items] |  |
Held-to-maturity, Amortized Cost, Totals | $ 599,096 | $ 562,434
Held-to-Maturity, Gross Unrealized Gains | 12,498 | 9,902
Held-to-Maturity, Gross Unrealized Losses | (1,043) | (3,405)
Held-to-Maturity, Fair Value | 610,551 | 568,931
States and Political Subdivisions [Member] |  |
Investment [Line Items] |  |
Held-to-maturity, Amortized Cost, Totals | 430,921 | 394,875
Held-to-Maturity, Gross Unrealized Gains | 10,435 | 7,622
Held-to-Maturity, Gross Unrealized Losses | (777) | (3,014)
Held-to-Maturity, Fair Value | 440,579 | 399,483
Corporate Bonds [Member] |  |
Investment [Line Items] |  |
Held-to-maturity, Amortized Cost, Totals | 129,143 | 143,858
Held-to-Maturity, Gross Unrealized Gains | 438 | 423
Held-to-Maturity, Gross Unrealized Losses | (176) | (265)
Held-to-Maturity, Fair Value | 129,405 | 144,016
Commercial Mortgage-Backed Securities [Member] |  |
Investment [Line Items] |  |
Held-to-maturity, Amortized Cost, Totals |  | 70
Held-to-Maturity, Fair Value |  | 70
U.S. Agency-Based Mortgage-Backed Securities [Member] |  |
Investment [Line Items] |  |
Held-to-maturity, Amortized Cost, Totals | 9,171 | 9,967
Held-to-Maturity, Gross Unrealized Gains | 797 | 948
Held-to-Maturity, Gross Unrealized Losses | (2) |
Held-to-Maturity, Fair Value | 9,966 | 10,915
U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member] |  |
Investment [Line Items] |  |
Held-to-maturity, Amortized Cost, Totals | 28,113 | 11,737
Held-to-Maturity, Gross Unrealized Gains | 689 | 746
Held-to-Maturity, Gross Unrealized Losses | (72) | (67)
Held-to-Maturity, Fair Value | 28,730 | 12,416
Asset-Backed Securities [Member] |  |
Investment [Line Items] |  |
Held-to-maturity, Amortized Cost, Totals | 1,748 | 1,927
Held-to-Maturity, Gross Unrealized Gains | 139 | 163
Held-to-Maturity, Gross Unrealized Losses | (16) | (59)
Held-to-Maturity, Fair Value | $ 1,871 | $ 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=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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