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Long-Term Investments - Additional Information (Detail) - USD ($) | 12 Months Ended |  |  |
--- | --- | --- | --- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014
Unrealized Losses On Short Term And Long Term Investments [Line Items] |  |  |  |
Minimum period for which failed auction securities been in unrealized loss position | 12 months |  |  |
Failed Auction Security [Member] |  |  |  |
Unrealized Losses On Short Term And Long Term Investments [Line Items] |  |  |  |
Amortized cost of securities | $ 3,000,000 | $ 3,000,000 |  |
Period for which failed auction securities been in unrealized loss position | Exceeds 12 months |  |  |
Estimated Fair Value | $ 2,525,000 | 2,508,000 |  |
Gross Unrealized Losses | 475,000 | 492,000 |  |
Aggregate credit loss | 48,000 | $ 59,000 | $ 72,000 | $ 84,000
Aggregate temporary impairment loss | $ 427,000 |  |  |

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- Definition Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt 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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- Definition Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 25 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=75030427&loc=d3e22054-111558 Reference 2: 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 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=82896770&loc=d3e26610-111562 + Details
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