Amount of specific reserve associated with troubled debt restructuring.

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Amounts charged-off on TDRs.

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Consumer Credit Exposures Period Past Due Required To Classify Loans As Special Mention Maximum.

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Financing receivables commitments to lend additional funds.

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Financing Receivables, Modifications, CARES Act

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Investment recorded represented by troubled debt restructuring in the current period.

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Loan participation interest held.

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Maximum exposure to credit risk of loans or receivables.

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Maximum potential amount of future undiscounted payments that could be required under the credit derivative, before reduction for potential recoveries under recourse or collateralization provisions.

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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.

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Carrying amount as of the balance sheet date of loans that have been pledged as collateral for borrowings. If the secured party has the right by contract or custom to sell or re-pledge the collateral, then the debtor reclassifies that asset and report that asset in its statement of financial position separately from other assets not so encumbered.

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Amount, before allowance for credit loss, of financing receivable.

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Amount, after allowance for credit loss, of financing receivable. Excludes financing receivable covered under loss sharing agreement.

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The aggregate amount of the participating mortgage obligation at the end of the accounting period.

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The sum of the carrying amounts, as of the date of the latest financial statement presented, of loans receivable which are owned but transferred to serve as collateral for the payment of related debt obligations, primarily secured borrowings and repurchase agreements, and for which the transferees are not permitted to sell or re-pledge them to an unrelated party.

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Loans and Allowance for Loan Losses - Narrative (Detail) - USD ($)	3 Months Ended		12 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Changes in lines of credit,resrticted to commercial exposure
Loan syndications, amount	$ 100,000,000
Mortgage loans partially or wholly-owned by others and serviced by the company	12,900,000		$ 13,500,000
Recorded investment	9,772,722,000		9,593,958,000
Total loans	$ 9,916,475,000	$ 9,087,103,000	9,730,525,000
Maximum number of days required for special mention	90 days
Amount of specific reserve associated with the TDR	$ 4,100,000		3,500,000
Amount of additional commitments to lend to borrowers who have been a party to a TDR	0		0
Outstanding recorded investment of loans that were new to troubled debt restructuring	300,000		3,900,000
Amounts charged-off on TDRs	0	400,000
Loan participation interest held	1,000,000,000.0		1,000,000,000.0
Payment Deferral \| CARES Act
Changes in lines of credit,resrticted to commercial exposure
Loan modifications	178,400,000		332,700,000
Commercial and industrial
Changes in lines of credit,resrticted to commercial exposure
Total loans	1,986,366,000	1,771,122,000	1,995,016,000
Business banking
Changes in lines of credit,resrticted to commercial exposure
Total loans	1,513,051,000	$ 779,916,000	1,339,164,000
Nonperforming Loans
Changes in lines of credit,resrticted to commercial exposure
Total loans	7,425,774,000		7,213,518,000
Nonperforming Loans \| Commercial and industrial
Changes in lines of credit,resrticted to commercial exposure
Total loans	1,986,366,000		1,995,016,000
Nonperforming Loans \| Business banking
Changes in lines of credit,resrticted to commercial exposure
Total loans	1,513,051,000		1,339,164,000
Unrated
Changes in lines of credit,resrticted to commercial exposure
Lines of credit, exposure	100,000
Unrated \| Nonperforming Loans
Changes in lines of credit,resrticted to commercial exposure
Total loans	1,770,861,000		1,580,852,000
Unrated \| Nonperforming Loans \| Commercial and industrial
Changes in lines of credit,resrticted to commercial exposure
Total loans	684,938,000		655,346,000
Unrated \| Nonperforming Loans \| Business banking
Changes in lines of credit,resrticted to commercial exposure
Total loans	1,079,605,000		918,921,000
Unrated \| Nonperforming Loans \| Paycheck Protection Program \| Commercial and industrial
Changes in lines of credit,resrticted to commercial exposure
Total loans	609,900,000		568,800,000
Unrated \| Nonperforming Loans \| Paycheck Protection Program \| Business banking
Changes in lines of credit,resrticted to commercial exposure
Total loans	628,200,000		457,400,000
Unrated \| Line of Credit \| Commercial and industrial
Changes in lines of credit,resrticted to commercial exposure
Lines of credit, exposure	1,000,000
Loss
Changes in lines of credit,resrticted to commercial exposure
Recorded investment	0
Loss \| Nonperforming Loans
Changes in lines of credit,resrticted to commercial exposure
Total loans	0		0
Loss \| Nonperforming Loans \| Commercial and industrial
Changes in lines of credit,resrticted to commercial exposure
Total loans	0		0
Loss \| Nonperforming Loans \| Business banking
Changes in lines of credit,resrticted to commercial exposure
Total loans	0		0
Federal Home Loan Bank Advances
Changes in lines of credit,resrticted to commercial exposure
Carrying value of loans pledged to secure advances from federal banks	2,400,000,000		2,400,000,000
Funds borrowed from federal banks	14,500,000		14,600,000
Federal Reserve Bank Advances
Changes in lines of credit,resrticted to commercial exposure
Carrying value of loans pledged to secure advances from federal banks	832,400,000		884,100,000
Funds borrowed from federal banks	$ 0		$ 0