Borrowings - Additional Information (Detail) - USD ($)	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Minimum contractual interest rates of long-term agreements to repurchase securities	4.09%	3.65%
Maximum contractual interest rates of long-term agreements to repurchase securities	4.58%	4.58%
Weighted-average contractual interest rates of long term agreements to repurchase securities	4.31%	4.05%
Repurchase dates of the long-term agreements outstanding	2020
Collateral posted	$ 442,000,000,000	$ 1,100,000,000
Debt Maturity, Start Year	Jan. 01, 2027
Debt Maturity, End Year	Dec. 31, 2033
Minimum [Member]
Debt Instrument [Line Items]
Variable interest rate of Senior Notes /Junior Subordinated Debentures	2.23%	1.74%
Maximum [Member]
Debt Instrument [Line Items]
Variable interest rate of Senior Notes /Junior Subordinated Debentures	4.71%	4.23%
FHLBs Facility [Member]
Debt Instrument [Line Items]
Borrowing facilities available with FHLB's amount	$ 20,200,000,000
Federal Reserve Bank of New York [Member]
Debt Instrument [Line Items]
Line of credit facility	$ 12,000,000,000
Variable Rate Senior Notes [Member]
Debt Instrument [Line Items]
Weighted-average variable rates payable		1.22%
Variable Rate Senior Notes [Member] \| Minimum [Member]
Debt Instrument [Line Items]
Variable interest rate of Senior Notes /Junior Subordinated Debentures	2.05%	1.18%
Variable Rate Senior Notes [Member] \| Maximum [Member]
Debt Instrument [Line Items]
Variable interest rate of Senior Notes /Junior Subordinated Debentures	2.05%	1.26%
Long-term Fixed Rate Advances From FHLB [Member]
Debt Instrument [Line Items]
Advances from the FHLB maturity date	2035
Long-term Fixed Rate Advances From FHLB [Member] \| Minimum [Member]
Debt Instrument [Line Items]
Variable contractual interest rate of long-term debt	1.97%	1.17%
Long-term Fixed Rate Advances From FHLB [Member] \| Maximum [Member]
Debt Instrument [Line Items]
Variable contractual interest rate of long-term debt	5.98%	7.32%
Long-term Fixed Rate Advances From FHLB [Member] \| Weighted Average [Member]
Debt Instrument [Line Items]
Variable contractual interest rate of long-term debt	2.06%	1.65%
Subordinated Capital Notes [Member]
Debt Instrument [Line Items]
Interest rate of debt instrument	2.78%	1.97%
Subordinated Variable Rate Notes Due 2021 [Member]
Debt Instrument [Line Items]
Interest rate of debt instrument	2.12%	1.57%
Junior Subordinated Debt [Member]
Debt Instrument [Line Items]
Weighted-average variable rates payable	2.83%	2.33%

Maximum contractual interest rates of long-term agreements to repurchase securities.

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Minimum contractual interest rates of long-term agreements to repurchase securities.

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Latest year in which includes a repurchase date to repurchase securities resulting from long term agreements.

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Weighted average contractual interest rates of long term agreements to repurchase securities.

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- Definition

Effective interest rate for the funds borrowed under the debt agreement considering interest compounding and original issue discount or premium.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(1))
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-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
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Contractual interest rate for funds borrowed, under the debt agreement.

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Latest date the outstanding debt instruments are required to be repaid, in CCYY-MM-DD format.

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Earliest date the outstanding debt instruments are required to be repaid, in CCYY-MM-DD format.

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Weighted average interest rate of debt outstanding.

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- Definition

The last year of maturity for the advances from the Federal Home Loan Bank as of the Balance Sheet date.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 470
-Section 50
-Paragraph 3
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=75038535&loc=d3e64711-112823

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States the interest rate applicable to Federal Home Loan Bank advances outstanding as of the Balance Sheet date.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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- Definition

Amount of current borrowing capacity under the credit facility considering any current restrictions on the amount that could be borrowed (for example, borrowings may be limited by the amount of current assets), but without considering any amounts currently outstanding under the facility.

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Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

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- Definition

Fair value of securities pledged as collateral against securities sold under agreement to repurchase.

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
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