Investment Securities - Summary of Remaining Contractual Principal Maturities for Available-for-Sale Securities (Details) $ in Thousands	9 Months Ended
Sep. 30, 2019 USD ($)
Debt Securities, Available-for-sale, Fair Value, Fiscal Year Maturity [Abstract]
Total	$ 12,866,857
One Year or Less	2,007,267
After One Year to Five Years	1,719,163
After Five Years to Ten Years	3,020,604
After Ten Years	$ 6,119,823
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	$ 6,334,848
One Year or Less	2,007,267
After One Year to Five Years	1,710,316
After Five Years to Ten Years	2,617,265
After Ten Years	0
Available-for-sale Securities \| U.S. agency debentures
Debt Securities, Available-for-sale, Fair Value, Fiscal Year Maturity [Abstract]
Total	100,000
One Year or Less	0
After One Year to Five Years	0
After Five Years to Ten Years	100,000
After Ten Years	0
Available-for-sale Securities \| Foreign government debt securities
Debt Securities, Available-for-sale, Fair Value, Fiscal Year Maturity [Abstract]
Total	8,847
One Year or Less	0
After One Year to Five Years	8,847
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	4,148,700
One Year or Less	0
After One Year to Five Years	0
After Five Years to Ten Years	0
After Ten Years	4,148,700
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	1,679,664
One Year or Less	0
After One Year to Five Years	0
After Five Years to Ten Years	3,131
After Ten Years	1,676,533
Available-for-sale Securities \| Agency-issued commercial mortgage-backed securities
Debt Securities, Available-for-sale, Fair Value, Fiscal Year Maturity [Abstract]
Total	594,798
One Year or Less	0
After One Year to Five Years	0
After Five Years to Ten Years	300,208
After Ten Years	$ 294,590

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 latest fiscal year. Excludes interim and annual periods when interim period is reported on rolling approach, from latest statement of financial position date.

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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 latest fiscal year. Excludes interim and annual periods when interim period is reported on rolling approach, from latest statement of financial position date.

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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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