Credit Quality And Allowance For Credit Losses Credit Quality And Allowance For Credit Losses (Troubled Debt Restructuring Subsequent Default) (Details) - USD ($) $ in Millions		Mar. 31, 2019	Mar. 31, 2018
Principal Deferrals
Financing Receivable, Modifications [Line Items]
Balance		$ 29	$ 131
Principal Deferrals | Business loans | Commercial borrower
Financing Receivable, Modifications [Line Items]
Balance		28	129
Principal Deferrals | Business loans | Commercial borrower | Domestic loans
Financing Receivable, Modifications [Line Items]
Balance		26	90
Principal Deferrals | Business loans | Commercial mortgage | Commercial borrower | Domestic loans
Financing Receivable, Modifications [Line Items]
Balance		2	39
Principal Deferrals | Business loans | Commercial mortgage | Commercial borrower | Domestic loans | Commercial Real Estate business line
Financing Receivable, Modifications [Line Items]
Balance	[1]	0	37
Principal Deferrals | Business loans | Commercial mortgage | Commercial borrower | Domestic loans | Other business lines
Financing Receivable, Modifications [Line Items]
Balance	[2]	2	2
Principal Deferrals | Retail loans | Home equity | Consumer borrower
Financing Receivable, Modifications [Line Items]
Balance	[3]	1	2
Interest Rate Reductions
Financing Receivable, Modifications [Line Items]
Balance		$ 4	$ 17

[1] Primarily loans to real estate developers.

[2] Primarily loans secured by owner-occupied real estate.

[3] Includes bankruptcy loans for which the court has discharged the borrower's obligation and the borrower has not reaffirmed the debt.