Investment Securities - Summary of Remaining Contractual Principal Maturities for Available-for-Sale Securities (Details) $ in Millions	3 Months Ended
Jun. 30, 2021 USD ($)
Debt Securities, Available-for-sale, Fair Value, Fiscal Year Maturity [Abstract]
Total	$ 23,876
One Year or Less	56
After One Year to Five Years	6,037
After Five Years to Ten Years	1,797
After Ten Years	15,986
U.S. treasury securities
Debt Securities, Available-for-sale, Fair Value, Fiscal Year Maturity [Abstract]
Total	6,235
One Year or Less	41
After One Year to Five Years	6,037
After Five Years to Ten Years	157
After Ten Years	0
U.S. agency debentures
Debt Securities, Available-for-sale, Fair Value, Fiscal Year Maturity [Abstract]
Total	81
One Year or Less	0
After One Year to Five Years	0
After Five Years to Ten Years	81
After Ten Years	0
Foreign government debt securities
Debt Securities, Available-for-sale, Fair Value, Fiscal Year Maturity [Abstract]
Total	15
One Year or Less	15
After One Year to Five Years	0
After Five Years to Ten Years	0
After Ten Years	0
Residential mortgage-backed securities \| Agency-issued mortgage-backed securities
Debt Securities, Available-for-sale, Fair Value, Fiscal Year Maturity [Abstract]
Total	11,562
One Year or Less	0
After One Year to Five Years	0
After Five Years to Ten Years	0
After Ten Years	11,562
Residential mortgage-backed securities \| Agency-issued collateralized mortgage obligations—fixed rate
Debt Securities, Available-for-sale, Fair Value, Fiscal Year Maturity [Abstract]
Total	4,424
One Year or Less	0
After One Year to Five Years	0
After Five Years to Ten Years	0
After Ten Years	4,424
Agency-issued commercial mortgage-backed securities
Debt Securities, Available-for-sale, Fair Value, Fiscal Year Maturity [Abstract]
Total	1,559
One Year or Less	0
After One Year to Five Years	0
After Five Years to Ten Years	1,559
After Ten Years	$ 0
Available-for-sale Securities \| Lower Limit
Debt Securities, Available-for-sale [Line Items]
Mortgage-backed securities contractual maturities (in years)	10 years
Available-for-sale Securities \| Upper Limit
Debt Securities, Available-for-sale [Line Items]
Mortgage-backed securities contractual maturities (in years)	30 years

The period (in years) that is typical for contractual maturities of mortgage-backed securities.

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Fair value of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), with single maturity date and allocated without single maturity date, maturing in sixth through tenth fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

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Fair value of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), with single maturity date and allocated without single maturity date, maturing in second through fifth fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

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Fair value of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), with single maturity date and allocated without single maturity date, maturing after tenth fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

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Fair value of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), with single maturity date and allocated without single maturity date, maturing in next fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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