Investment In Associated Companies - Additional Information (Detail) - USD ($)	6 Months Ended	12 Months Ended
Jun. 30, 2018	Dec. 31, 2017
Seadrill Partners LLC [Member]
Schedule of Investments [Line Items]
Equity method investment other than temporary impairment	$ 0
Remaining economic lives of underlying assets	30 years
Weighted average cost of capital	9.75%
Total Cost method investments other than temporary impairment	$ 0
SeaMex Limited [Member]
Schedule of Investments [Line Items]
Equity method investment other than temporary impairment	$ 0	$ 36,000,000
Remaining economic lives of underlying assets	30 years
Weighted average cost of capital	10.25%
Ownership interest	50.00%
Direct Ownership Interest [Member] \| Seadrill Partners LLC [Member]
Schedule of Investments [Line Items]
Equity method investment other than temporary impairment		82,000,000
Subordinated Units [Member] \| Seadrill Partners LLC [Member]
Schedule of Investments [Line Items]
Equity method investment other than temporary impairment		$ 723,000,000
Non Economic Interest [Member] \| Seadrill Partners LLC [Member]
Schedule of Investments [Line Items]
Ownership interest	0.00%
Incentive Distribution Rights [Member] \| Seadrill Partners LLC [Member]
Schedule of Investments [Line Items]
Ownership interest	100.00%

Cost Method Investment, Ownership Percentage

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This item represents an other than temporary decline in value that has been recognized against an investment accounted for under the cost method of accounting. The excess of the carrying amount over the fair value of the investment represents the amount of the write down which is or was reflected in earnings. The written down value is a new cost basis with the adjusted value of the investment becoming its new carrying value. Evidence of a loss in value might include, but would not necessarily be limited to, absence of an ability to recover the carrying amount of the investment or inability of the investee to sustain an earnings capacity which would justify the carrying amount of the investment.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 325
-SubTopic 20
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=75025870&loc=d3e40691-111596

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This item represents an other than temporary decline in value that has been recognized against an investment accounted for under the equity method of accounting. The excess of the carrying amount over the fair value of the investment represents the amount of the write down which is or was reflected in earnings. The written down value is a new cost basis with the adjusted value of the investment becoming its new carrying value subject to the equity accounting method. Evidence of a loss in value might include, but would not necessarily be limited to, absence of an ability to recover the carrying amount of the investment or inability of the investee to sustain an earnings capacity which would justify the carrying amount of the investment.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 323
-SubTopic 10
-Section 35
-Paragraph 32
-URI http://asc.fasb.org/extlink&oid=99375318&loc=d3e32787-111569

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The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 323
-SubTopic 10
-Section 50
-Paragraph 3
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Interest rate used to find the present value of an amount to be paid or received in the future as an input to measure fair value. For example, but not limited to, weighted average cost of capital (WACC), cost of capital, cost of equity and cost of debt.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
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Threshold period for investment in equity securities in a continuous decline in value to be considered other than temporary impairment (OTTI), in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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