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Investments - Additional Information (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2018 | Sep. 30, 2017 | Sep. 30, 2018 | Sep. 30, 2017
Investments Schedule [Abstract] |  |  |  |
Realized gain (loss) on marketable securities | $ 0.6 | $ 0.3 | $ 1.9 | $ (1.6)

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- References No definition available. + Details
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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_
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Balance Type: | credit
Period Type: | duration