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Loss on impairment of long-lived assets (Details) $ in Millions | 6 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2018 USD ($) | Jul. 01, 2018 USD ($) rig | Dec. 31, 2020 USD ($) rig | Dec. 31, 2019 USD ($)
Impaired Long-Lived Assets Held and Used [Line Items] |  |  |  |
Number of impaired rigs | rig |  | 3 | 15 |
Loss on impairment of long-lived assets | $ 0 | $ 414 | $ 4,087 | $ 0
Drilling units |  |  |  |
Impaired Long-Lived Assets Held and Used [Line Items] |  |  |  |
Loss on impairment of long-lived assets |  |  | $ 4,087 |

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- Definition The aggregate amount of write-downs for impairments recognized during the period for long lived assets held for use (including those held for disposal by means other than sale). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3602-108585 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 360 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=109226691&loc=d3e2921-110230 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 360 -SubTopic 10 -Section 45 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=109226348&loc=d3e2420-110228 + Details
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