Amount after accumulated depreciation, depletion and amortization of physical assets used in the normal conduct of business to produce goods and services 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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Name:	us-gaap_PropertyPlantAndEquipmentNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Leases - Sale and Leased-backed asset and obligation (Details) - USD ($) $ in Millions	Dec. 31, 2020	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019
Property, plant and equipment - at cost
Finance lease asset	$ 9,421		$ 9,333
Finance lease liability	$ 115		$ 132
Constructed machinery and equipment \| Municipal securities
Property, plant and equipment - at cost
Finance lease asset				$ 9
Finance lease liability		$ 10		$ 9