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Marketable Securities - Additional Information (Detail) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Investments, Debt and Equity Securities [Abstract] |  |  |
Net realized losses on investments | $ (1) |  |
Realized investment gains (losses), description |  | Realized gains and losses on sales of investments were not material in 2017 and 2016. | Realized gains and losses on sales of investments were not material in 2017 and 2016.

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- References No definition available. + Details
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- Definition The net realized gains or losses on investments during the period, not including gains or losses on securities separately or otherwise categorized as trading, available-for-sale, or held-to-maturity. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 220 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04.3(a)) -URI http://asc.fasb.org/extlink&oid=114873790&loc=SL114874131-224263 + Details
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- Definition Realized Investment Gains Losses Description + References No definition available. + Details
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