Derivatives, Hedging Activities and Interest Expense - Additional Information (Details) - USD ($)	Sep. 30, 2018	Mar. 31, 2018
Derivatives Fair Value [Line Items]
Derivative assets and liabilities used for hedge accounting	$ 0
Collateral held	366,000,000	$ 737,000,000
Counterparty netting and collateral	923,000,000	262,000,000
Counterparty netting, derivative assets	579,000,000	560,000,000
Counterparty netting, derivative liabilities	579,000,000	560,000,000
Other liabilities [Member]
Derivatives Fair Value [Line Items]
Collateral held in excess of the fair value of derivative assets	24,000,000	11,000,000
Other assets [Member]
Derivatives Fair Value [Line Items]
Collateral held		1,297,000,000
Collateral posted in excess of the fair value of derivative liabilities	$ 9,000,000	$ 3,000,000

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Fair value of liability associated with financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset, and obligation to return cash collateral under master netting arrangements.

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Amount of obligation to return cash collateral under master netting arrangements that have not been offset against derivative assets.

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Aggregate net fair value of all derivative instruments designated as hedging instruments. Includes instruments designated as cash flow hedges, fair value hedges, and hedges of net investments in foreign operations.

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