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Available-for-sale Securities and Fair Value on Financial Instruments - Additional Information (Detail) - USD ($) | 3 Months Ended
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Mar. 31, 2019 | Mar. 31, 2018
Investments Debt And Equity Securities [Abstract] |  |
Other-than-temporary impairment losses | $ 0 | $ 0
Gross realized gains from the sale or maturity of available-for-sale investments | 0 | 0
Gross realized losses from the sale or maturity of available-for-sale investments | $ 0 | $ 0

X
- Definition This item represents the gross profit realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities. + References No definition available. + Details
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X
- Definition This item represents the gross loss realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities. + References No definition available. + Details
Name: | us-gaap_AvailableForSaleSecuritiesGrossRealizedLosses
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Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition The amount by which the fair value of an investment is less than the amortized cost basis or carrying amount of that investment at the balance sheet date and the decline in fair value is deemed to be other than temporary, before considering whether or not such amount is recognized in earnings or other comprehensive income. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 8A -URI http://asc.fasb.org/extlink&oid=109979856&loc=SL6284422-111562 + Details
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- References No definition available. + Details
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