Investment Securities - Summary of Remaining Contractual Principal Maturities for Available-for-Sale Securities (Detail) $ in Thousands	9 Months Ended
Sep. 30, 2018 USD ($)
Debt Securities, Available-for-sale, Fair Value, Fiscal Year Maturity [Abstract]
Total	$ 9,087,609
One Year or Less	2,669,709
After One Year to Five Years	3,741,021
After Five Years to Ten Years	466,929
After Ten Years	2,209,950
U.S. treasury securities
Debt Securities, Available-for-sale, Fair Value, Fiscal Year Maturity [Abstract]
Total	5,501,874
U.S. agency debentures
Debt Securities, Available-for-sale, Fair Value, Fiscal Year Maturity [Abstract]
Total	1,346,672
Residential mortgage-backed securities \| Fixed rate \| Agency-issued collateralized mortgage obligations
Debt Securities, Available-for-sale, Fair Value, Fiscal Year Maturity [Abstract]
Total	1,920,541
Residential mortgage-backed securities \| Variable rate \| Agency-issued collateralized mortgage obligations
Debt Securities, Available-for-sale, Fair Value, Fiscal Year Maturity [Abstract]
Total	$ 318,522
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]
One Year or Less	$ 2,016,438
After One Year to Five Years	3,047,620
After Five Years to Ten Years	437,816
After Ten Years	0
Available-for-sale Securities \| U.S. agency debentures
Debt Securities, Available-for-sale, Fair Value, Fiscal Year Maturity [Abstract]
One Year or Less	653,271
After One Year to Five Years	693,401
After Five Years to Ten Years	0
After Ten Years	0
Available-for-sale Securities \| Residential mortgage-backed securities \| Fixed rate \| Agency-issued collateralized mortgage obligations
Debt Securities, Available-for-sale, Fair Value, Fiscal Year Maturity [Abstract]
One Year or Less	0
After One Year to Five Years	0
After Five Years to Ten Years	29,113
After Ten Years	1,891,428
Available-for-sale Securities \| Residential mortgage-backed securities \| Variable rate \| Agency-issued collateralized mortgage obligations
Debt Securities, Available-for-sale, Fair Value, Fiscal Year Maturity [Abstract]
One Year or Less	0
After One Year to Five Years	0
After Five Years to Ten Years	0
After Ten Years	$ 318,522

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), 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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