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Investments in Marketable Securities - Additional Information (Details) - USD ($) | 3 Months Ended | 9 Months Ended |
--- | --- | --- | ---
Sep. 30, 2018 | Sep. 30, 2017 | Sep. 30, 2018 | Sep. 30, 2017 | Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract] |  |  |  |  |
Investments in marketable securities | $ 144,100,000 |  | $ 144,100,000 |  | $ 77,800,000
Proceeds received from sales of marketable securities | 16,600,000 | $ 23,600,000 | 42,600,000 | $ 81,900,000 |
Net realized gains (losses) on sale of marketable securities | $ 0 | $ 9,100,000 | $ (4,000,000) | $ 24,200,000 |

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- Definition Amount of investment in marketable security. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.4) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 + Details
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- Definition Amount of realized gain (loss) on investment in marketable security, including other-than-temporary impairment (OTTI). + References No definition available. + Details
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- Definition The cash inflow associated with the aggregate amount received by the entity through sale or maturity of marketable securities (held-to-maturity or available-for-sale) during the period. + References No definition available. + Details
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