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

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).

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
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Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
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