Amount of accumulated depreciation, depletion and amortization for physical assets used in the normal conduct of business to produce goods and services.

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The aggregate expense recognized in the current period that allocates the cost of tangible assets, intangible assets, or depleting assets to periods that benefit from use of the assets.

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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).

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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Amount of acquisition of long-lived, physical assets used in the normal conduct of business and not intended for resale. Examples include, but are not limited to, land, buildings, machinery and equipment, office equipment, furniture and fixtures, and computer equipment.

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Drilling units (Details) - USD ($) $ in Millions	6 Months Ended
Drilling units (Details) - USD ($) $ in Millions	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018	Dec. 31, 2017
Cost
Impairment	$ 0
Predecessor
Cost
Impairment		$ (414)
Drilling units
Cost
Opening balance	6,890
Additions	61
Closing balance	6,951
Accumulated depreciation
Opening balance	(231)
Depreciation	(207)
Closing balance	(438)
Net book value	$ 6,513		$ 6,659
Drilling units \| Predecessor
Cost
Opening balance		17,335
Additions		117
Impairment		(414)
Closing balance		17,038
Accumulated depreciation
Opening balance		(4,119)
Depreciation		(388)
Closing balance		(4,507)
Net book value		$ 12,531		$ 13,216