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Marketable Securities - Net Unrealized Gains and Losses for Consolidated Statement of Operations (Detail) - USD ($) $ in Millions | 6 Months Ended
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Jun. 30, 2018 | Jun. 30, 2017
Marketable Securities [Line Items] |  |
Total unrealized (loss) / gain on marketable securities | $ (3) | $ 6
Seadrill Partners Common Units [Member] |  |
Marketable Securities [Line Items] |  |
Total unrealized (loss) / gain on marketable securities | (5) | (21)
Archer [Member] |  |
Marketable Securities [Line Items] |  |
Total unrealized (loss) / gain on marketable securities | $ 2 | $ 27

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- Definition Marketable Securities [Line Items] + References No definition available. + Details
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- Definition The aggregate net change in the difference between the fair value and the carrying value, or in the comparative fair values, of marketable securities categorized as trading held at each balance sheet date, that was included in earnings for the period, which may have arisen from (a) securities classified as trading, (b) the unrealized holding gain (loss) on held-to-maturity securities transferred to the trading security category, and (c) the cumulative unrealized gain (loss) which was included in other comprehensive income (a separate component of shareholders' equity) on available-for-sale securities transferred to trading securities during the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 9 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27357-111563 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 10 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27405-111563 + Details
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