Drilling units (Details) - Drilling rigs - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Cost
Opening balance	$ 6,714.0	$ 6,599.0
Additions	111.1	115.0
Closing balance	6,825.1	6,714.0	$ 6,599.0
Accumulated depreciation
Opening balance	(1,708.4)	(1,428.1)
Depreciation	(275.9)	(280.3)	(274.9)
Closing balance	(1,984.3)	(1,708.4)	(1,428.1)
Net Book Value	4,840.8	5,005.6	5,170.9
Depreciation	$ (275.9)	$ (280.3)	$ (274.9)

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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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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Amount before accumulated depreciation, depletion and amortization of 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, and furniture and fixtures.

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