Investment Securities - Summary of Remaining Contractual Principal Maturities for Available-for-Sale Securities (Details) $ in Thousands	3 Months Ended
Jun. 30, 2020 USD ($)
Debt Securities, Available-for-sale, Fair Value, Fiscal Year Maturity [Abstract]
Total	$ 18,451,913
One Year or Less	108,043
After One Year to Five Years	2,613,833
After Five Years to Ten Years	3,286,269
After Ten Years	$ 12,443,768
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
Available-for-sale Securities \| U.S. treasury securities
Debt Securities, Available-for-sale, Fair Value, Fiscal Year Maturity [Abstract]
Total	$ 4,535,235
One Year or Less	85,518
After One Year to Five Years	2,613,342
After Five Years to Ten Years	1,836,375
After Ten Years	0
Available-for-sale Securities \| U.S. agency debentures
Debt Securities, Available-for-sale, Fair Value, Fiscal Year Maturity [Abstract]
Total	102,659
One Year or Less	0
After One Year to Five Years	0
After Five Years to Ten Years	102,659
After Ten Years	0
Available-for-sale Securities \| Foreign government debt securities
Debt Securities, Available-for-sale, Fair Value, Fiscal Year Maturity [Abstract]
Total	22,525
One Year or Less	22,525
After One Year to Five Years	0
After Five Years to Ten Years	0
After Ten Years	0
Available-for-sale Securities \| Residential mortgage-backed securities \| Agency-issued mortgage-backed securities
Debt Securities, Available-for-sale, Fair Value, Fiscal Year Maturity [Abstract]
Total	7,655,556
One Year or Less	0
After One Year to Five Years	0
After Five Years to Ten Years	0
After Ten Years	7,655,556
Available-for-sale Securities \| Residential mortgage-backed securities \| Agency-issued collateralized mortgage obligations—fixed rate
Debt Securities, Available-for-sale, Fair Value, Fiscal Year Maturity [Abstract]
Total	2,979,540
One Year or Less	0
After One Year to Five Years	491
After Five Years to Ten Years	0
After Ten Years	2,979,049
Available-for-sale Securities \| Agency-issued commercial mortgage-backed securities
Debt Securities, Available-for-sale, Fair Value, Fiscal Year Maturity [Abstract]
Total	3,156,398
One Year or Less	0
After One Year to Five Years	0
After Five Years to Ten Years	1,347,235
After Ten Years	$ 1,809,163

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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