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Investments - Additional Information (Detail) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2018 | Jun. 30, 2017 | Jun. 30, 2018 | Jun. 30, 2017
Investments Schedule [Abstract] |  |  |  |
Realized gain (loss) on marketable securities | $ 1.7 | $ (1.8) | $ 1.3 | $ (1.9)

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- References No definition available. + Details
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Namespace Prefix: | invest_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Amount of realized gain (loss) on investment in debt and equity securities. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration