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Investments - Summary of Investments (Detail) - USD ($) $ in Thousands | Dec. 31, 2017 | Dec. 31, 2016
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Schedule of Available-for-sale Securities [Line Items] |  |
Adjusted Cost | $ 89,683 |
Gross Unrealized (Losses) | (412) |
Estimated Fair Value | 89,271 |
Certificates of Deposit [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Adjusted Cost | 4,915 |
Gross Unrealized (Losses) | (3) |
Estimated Fair Value | 4,912 |
Municipal Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Adjusted Cost | 84,768 | $ 62,769
Gross Unrealized Gains |  | 12
Gross Unrealized (Losses) | (409) | (185)
Estimated Fair Value | $ 84,359 | $ 62,596

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- Definition Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563 + Details
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- Definition Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563 + Details
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