Financial Derivative Instruments - Carrying Amount of Fair Value Hedges (Details) - USD ($)		Mar. 31, 2020	Mar. 31, 2019
Derivative [Line Items]
Deferred Gain (Loss) on Discontinuation of Interest Rate Fair Value Hedge		$ 18,000,000	$ 20,000,000
Designated as Hedging Instrument
Derivative [Line Items]
Derivative, Amount of Hedged Item		504,000,000	795,000,000
Short-term Debt | Designated as Hedging Instrument
Derivative [Line Items]
Derivative, Amount of Hedged Item		504,000,000	300,000,000
Long-term Debt | Designated as Hedging Instrument
Derivative [Line Items]
Derivative, Amount of Hedged Item		0	495,000,000
Cumulative fair value adjustment [Member] | Designated as Hedging Instrument
Derivative [Line Items]
Derivative, Amount of Hedged Item	[1]	22,000,000	15,000,000
Cumulative fair value adjustment [Member] | Short-term Debt | Designated as Hedging Instrument
Derivative [Line Items]
Derivative, Amount of Hedged Item	[1]	4,000,000	2,000,000
Cumulative fair value adjustment [Member] | Long-term Debt | Designated as Hedging Instrument
Derivative [Line Items]
Derivative, Amount of Hedged Item	[1]	$ 18,000,000	$ 13,000,000

[1] At March 31, 2020 and 2019, these amounts include the cumulative amount of fair value hedging adjustments remaining for which hedge accounting has been discontinued of $18 million and $20 million, respectively.