Derivative Financial Instruments - Information about Interest Rate Swap Agreements (Detail) - USD ($)	3 Months Ended	12 Months Ended
Mar. 31, 2018	Dec. 31, 2017
Derivative [Line Items]
Notional Amount	$ 9,550,000,000	$ 9,400,000,000
Average Maturity (in years)	2 years 6 months	2 years 6 months
Estimated Fair Value Gain	$ 376,000	$ 639,000
Interest Payments On Variable Rate Commercial Real Estate Loans [Member] \| Cash Flow Hedges [Member]
Derivative [Line Items]
Notional Amount	$ 4,850,000,000	$ 4,850,000,000
Average Maturity (in years)	1 year 8 months 12 days	2 years
Weighted-Average Rate, Fixed	1.52%	1.52%
Weighted-Average Rate, Variable	1.66%	1.36%
Estimated Fair Value Gain	$ 60,000	$ 66,000
Fixed Rate Long-Term Borrowings [Member] \| Fair Value Hedges [Member]
Derivative [Line Items]
Notional Amount	$ 4,700,000,000	$ 4,550,000,000
Average Maturity (in years)	3 years	2 years 10 months 25 days
Weighted-Average Rate, Fixed	2.41%	2.27%
Weighted-Average Rate, Variable	2.48%	2.09%
Estimated Fair Value Gain	$ 316,000	$ 573,000

Aggregate notional amount specified by the derivative(s). Expressed as an absolute value.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Article 12
-Section 13
-Sentence Column B

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- Definition

Average fixed interest rate related to the group of interest rate derivatives.

+ References

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- Definition

Average remaining period until maturity of the derivative contract, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

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- Definition

Average variable interest rate related to the group of interest rate derivatives.

+ References

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

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- Definition

Amount of unrealized gain (loss) related to the fair value of interest rate derivatives designated as fair value hedging instruments, as offset by the gain (loss) on the hedged item to the extent that the fair value hedge is determined to be effective.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4A
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5618551-113959

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4C
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5624171-113959

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