Acquisitions, Development and Divestitures - Discontinued Operations (Details) £ in Thousands, $ in Millions			3 Months Ended
	Jul. 31, 2021 USD ($) Rate	Jul. 31, 2021 GBP (£)	Mar. 31, 2021 USD ($)	Mar. 31, 2020 USD ($)	Jul. 31, 2021 GBP (£) Rate	Dec. 31, 2020 USD ($)
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Income (Loss) from Discontinued Operations (net of income taxes)			$ (2,043)	$ 350
Disposal Group, Including Discontinued Operation, Balance Sheet Disclosures [Abstract]
Total Assets			18,425			$ 18,983
Total Liabilities			11,376			11,023
Discontinued Operations, Held-for-sale | WPD
Discontinued Operation, Income (Loss) from Discontinued Operation Disclosures [Abstract]
Loss on Sale			(1,647)	0
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI for Sale of Disposal Group, Net of Tax			3,700
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Operating Revenues			634	614
Operating Expenses			252	222
Other Income (Expense) - net			66	130
Interest Expense			93	93
Income before income taxes			355	429
Loss on Sale			(1,647)	0
Income tax expense			751	79
Income (Loss) from Discontinued Operations (net of income taxes)			(2,043)	350
Disposal Group, Including Discontinued Operation, Income Tax Expense Attributable to Realization of Book-Tax Outside Basis, Domestic			689
Disposal Group, Including Discontinued Operation, Income Tax Expense Attributable to Realization of Book-Tax Outside Basis, Foreign			73
Disposal Group, Including Discontinued Operation, Balance Sheet Disclosures [Abstract]
Cash and cash equivalents			152			266
Accounts receivable and unbilled revenues			459			389
Price risk management assets			62			146
Property, plant and equipment, net			15,236			14,392
Goodwill			1,029			2,558
Other intangibles			434			413
Pension benefit asset			911			682
Other assets			142			137
Total Assets			18,425			18,983
Short-term debt and long-term debt due within one year			812			994
Accounts payable			199			220
Customer deposits			240			217
Accrued interest			178			190
Long-term debt			8,297			7,938
Total deferred income taxes			1,123			1,032
Price risk management liabilities			180			137
Other deferred credits and liabilities			347			295
Total Liabilities			11,376			11,023
Net assets			7,049			$ 7,960
Loss on Sale			(1,647)	$ 0
Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Gain (Loss), before Reclassification and Tax			118
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI for Sale of Disposal Group, Net of Tax			$ 3,700
Discontinued Operations, Held-for-sale | WPD | Forecast [Member]
Discontinued Operation, Income (Loss) from Discontinued Operation Disclosures [Abstract]
Sales proceeds	$ 10,806	£ 7,800,000
Proceeds from Daily Dividend, Divestiture of Business | £					£ 548
Loss on Sale	(1,647)
Costs to sell	48
Carrying value	12,405
Additional Proceeds From Divestiture Of Business Not Included In Selling Price | £		£ 49,000
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI for Sale of Disposal Group, Net of Tax	$ 3,700
Foreign Currency Exchange Rate, Translation | Rate	138.00%				138.00%
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Loss on Sale	$ (1,647)
Disposal Group, Including Discontinued Operation, Balance Sheet Disclosures [Abstract]
Loss on Sale	(1,647)
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI for Sale of Disposal Group, Net of Tax	$ 3,700