Income Taxes - Deferred Tax Assets (Liabilities) (Details) - USD ($) $ in Millions	Mar. 31, 2021	Mar. 31, 2020
Deferred tax assets
Investment basis differences	$ 32	$ 0
Tax loss/credit carryforwards	4,039	2,516
Accrued interest	20	12
Operating lease liabilities	359	370
Contract accounting	92	126
Other assets	351	144
Total deferred tax assets	4,893	3,168
Valuation allowance	(3,860)	(2,162)
Net deferred tax assets	1,033	1,006
Deferred tax liabilities
Depreciation and amortization	(513)	(850)
Operating right-of-use asset	(339)	(343)
Investment basis differences	0	(68)
Employee benefits	(6)	(48)
Other liabilities	(246)	(150)
Total deferred tax liabilities	(1,104)	(1,459)
Total net deferred tax assets (liabilities)	$ (71)	$ (453)