Accumulated Other Comprehensive Income (Loss) Accumulated Other Comprehensive Income (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Amounts reclassified from accumulated other comprehensive income (loss)	$ (1)	$ (25)	$ (6)	$ (35)
Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Amounts reclassified from accumulated other comprehensive income (loss)	$ (1)	$ (24)	$ (6)	$ (34)