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, 2017 USD ($)
Available-for-sale Securities
Investments Classified by Contractual Maturity Date [Line Items]
Available-for-sale Securities, Debt Securities	$ 12,379,813
Available-for-sale Securities, Debt Maturities, Next Twelve Months, Fair Value	2,821,471
Available-for-sale Securities, Debt Maturities, Year Two Through Five, Fair Value	7,462,432
Available-for-sale Securities, Debt Maturities, Year Six Through Ten, Fair Value	622,312
Available-for-sale Securities, Debt Maturities, after Ten Years, Fair Value	1,473,598
Available-for-sale Securities \| U.S. treasury securities
Investments Classified by Contractual Maturity Date [Line Items]
Available-for-sale Securities, Debt Securities	8,206,090
Available-for-sale Securities, Debt Maturities, Next Twelve Months, Fair Value	2,321,436
Available-for-sale Securities, Debt Maturities, Year Two Through Five, Fair Value	5,884,654
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	2,077,813
Available-for-sale Securities, Debt Maturities, Next Twelve Months, Fair Value	500,035
Available-for-sale Securities, Debt Maturities, Year Two Through Five, Fair Value	1,577,778
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	1,650,329
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	622,312
Available-for-sale Securities, Debt Maturities, after Ten Years, Fair Value	1,028,017
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	445,581
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	$ 445,581
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

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 320
-Section 50
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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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