Investment Securities - Summary of Remaining Contractual Principal Maturities and Fully Taxable Equivalent Yields on Debt Securities Classified as Available for Sale (Detail) $ in Thousands	3 Months Ended
Mar. 31, 2018 USD ($)
Available-for-sale Securities
Investments Classified by Contractual Maturity Date [Line Items]
Available-for-sale Securities, Debt Securities	$ 10,080,384
Available-for-sale Securities, Debt Maturities, Next Twelve Months, Fair Value	2,719,423
Available-for-sale Securities, Debt Maturities, Year Two Through Five, Fair Value	4,859,288
Available-for-sale Securities, Debt Maturities, Year Six Through Ten, Fair Value	66,459
Available-for-sale Securities, Debt Maturities, after Ten Years, Fair Value	2,435,214
Available-for-sale Securities \| U.S. treasury securities
Investments Classified by Contractual Maturity Date [Line Items]
Available-for-sale Securities, Debt Securities	6,016,877
Available-for-sale Securities, Debt Maturities, Next Twelve Months, Fair Value	2,213,143
Available-for-sale Securities, Debt Maturities, Year Two Through Five, Fair Value	3,803,734
Available-for-sale Securities, Debt Maturities, Year Six Through Ten, Fair Value	0
Available-for-sale Securities, Debt Maturities, after Ten Years, Fair Value	0
Available-for-sale Securities \| U.S. agency debentures
Investments Classified by Contractual Maturity Date [Line Items]
Available-for-sale Securities, Debt Securities	1,561,834
Available-for-sale Securities, Debt Maturities, Next Twelve Months, Fair Value	506,280
Available-for-sale Securities, Debt Maturities, Year Two Through Five, Fair Value	1,055,554
Available-for-sale Securities, Debt Maturities, Year Six Through Ten, Fair Value	0
Available-for-sale Securities, Debt Maturities, after Ten Years, Fair Value	0
Available-for-sale Securities \| Residential mortgage-backed securities \| Agency-issued collateralized mortgage obligations \| Fixed rate
Investments Classified by Contractual Maturity Date [Line Items]
Available-for-sale Securities, Debt Securities	2,146,173
Available-for-sale Securities, Debt Maturities, Next Twelve Months, Fair Value	0
Available-for-sale Securities, Debt Maturities, Year Two Through Five, Fair Value	0
Available-for-sale Securities, Debt Maturities, Year Six Through Ten, Fair Value	66,459
Available-for-sale Securities, Debt Maturities, after Ten Years, Fair Value	2,079,714
Available-for-sale Securities \| Residential mortgage-backed securities \| Agency-issued collateralized mortgage obligations \| Variable rate
Investments Classified by Contractual Maturity Date [Line Items]
Available-for-sale Securities, Debt Securities	355,500
Available-for-sale Securities, Debt Maturities, Next Twelve Months, Fair Value	0
Available-for-sale Securities, Debt Maturities, Year Two Through Five, Fair Value	0
Available-for-sale Securities, Debt Maturities, Year Six Through Ten, Fair Value	0
Available-for-sale Securities, Debt Maturities, after Ten Years, Fair Value	$ 355,500
Lower Limit \| Held-to-maturity Securities [Member]
Investments Classified by Contractual Maturity Date [Line Items]
Mortgage-backed securities contractual maturities (in years)	10 years
Upper Limit \| Held-to-maturity Securities [Member]
Investments Classified by Contractual Maturity Date [Line Items]
Mortgage-backed securities contractual maturities (in years)	30 years

Investments Classified by Contractual Maturity Date [Line Items]

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

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Fair value of available-for-sale debt securities maturing in the sixth fiscal year through the tenth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

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-Publisher FASB
-Name Accounting Standards Codification
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Fair value of available-for-sale debt securities maturing in the second fiscal year through the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Fair value of available-for-sale debt securities maturing after the tenth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Fair value of available-for-sale debt securities maturing in the next fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount of debt securities categorized neither as held-to-maturity nor trading.

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