Investment Securities - Summary of Remaining Contractual Principal Maturities for Available-for-Sale Securities (Details) $ in Thousands	6 Months Ended
Jun. 30, 2019 USD ($)
Debt Securities, Available-for-sale, Fair Value, Fiscal Year Maturity [Abstract]
Total	$ 7,940,322
One Year or Less	1,677,718
After One Year to Five Years	1,984,004
After Five Years to Ten Years	1,173,807
After Ten Years	$ 3,104,793
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	$ 4,821,074
One Year or Less	1,677,718
After One Year to Five Years	1,974,793
After Five Years to Ten Years	1,168,563
After Ten Years	0
Available-for-sale Securities \| Foreign government debt securities
Debt Securities, Available-for-sale, Fair Value, Fiscal Year Maturity [Abstract]
Total	9,211
One Year or Less	0
After One Year to Five Years	9,211
After Five Years to Ten Years	0
After Ten Years	0
Available-for-sale Securities \| Residential mortgage-backed securities
Debt Securities, Available-for-sale, Fair Value, Fiscal Year Maturity [Abstract]
After Five Years to 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	1,321,857
One Year or Less	0
After One Year to Five Years	0
After Ten Years	1,321,857
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	1,788,180
One Year or Less	0
After One Year to Five Years	0
After Five Years to Ten Years	5,244
After Ten Years	$ 1,782,936

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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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 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), maturing in next 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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