Derivative Financial Instruments - Information about Interest Rate Swap Agreements (Detail) - USD ($)	9 Months Ended	12 Months Ended
Sep. 30, 2020	Dec. 31, 2019
Derivative [Line Items]
Notional Amount	$ 54,550,000,000	$ 57,550,000,000
Average Maturity (in years)	1 year 1 month 6 days	1 year 6 months
Estimated Fair Value Gain (Loss)	$ 377,000	$ (1,762,000)
Interest Payments On Variable Rate Commercial Real Estate Loans [Member] \| Cash Flow Hedges [Member]
Derivative [Line Items]
Notional Amount	$ 52,250,000,000	$ 53,750,000,000
Average Maturity (in years)	1 year 1 month 6 days	1 year 4 months 24 days
Weighted-Average Rate, Fixed	2.40%	2.44%
Weighted-Average Rate, Variable	0.15%	1.73%
Estimated Fair Value Gain (Loss)	$ 890,000	$ (1,195,000)
Fixed Rate Long-Term Borrowings [Member] \| Fair Value Hedges [Member]
Derivative [Line Items]
Notional Amount	$ 2,300,000,000	$ 3,800,000,000
Average Maturity (in years)	2 years 8 months 12 days	2 years 2 months 12 days
Weighted-Average Rate, Fixed	2.80%	2.51%
Weighted-Average Rate, Variable	0.69%	2.27%
Estimated Fair Value Gain (Loss)	$ (513,000)	$ (567,000)

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

No definition available.

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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

No definition available.

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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

No definition available.

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- Definition

Nominal or face amount used to calculate payment on derivative.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1B
-URI http://asc.fasb.org/extlink&oid=121590274&loc=SL5580258-113959

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1A
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=121590274&loc=SL5579245-113959

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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://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4A
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