Loans and Leases and the Allowance for Credit Losses - Summary of Outstanding Loan Balances Related to COVID-19 Modifications Granted (Detail) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2021	Mar. 31, 2020
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases, net of unearned discount		$ 129,961	$ 66,517
Payment Deferral [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases, net of unearned discount		85,516	13,019
C O V I D19
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases, net of unearned discount		4,632,997
C O V I D19 | Payment Deferral [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases, net of unearned discount	[1]	4,013,941
C O V I D19 | Other Forbearances [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases, net of unearned discount	[2]	619,056
Commercial, Financial, Leasing, etc. [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases, net of unearned discount		53,157	12,053
Commercial, Financial, Leasing, etc. [Member] | Payment Deferral [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases, net of unearned discount		24,653	5,066
Commercial, Financial, Leasing, etc. [Member] | C O V I D19
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases, net of unearned discount		217,310
Commercial, Financial, Leasing, etc. [Member] | C O V I D19 | Payment Deferral [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases, net of unearned discount	[1]	60,819
Commercial, Financial, Leasing, etc. [Member] | C O V I D19 | Other Forbearances [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases, net of unearned discount	[2]	156,491
Commercial [Member] | C O V I D19
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases, net of unearned discount		869,155
Commercial [Member] | C O V I D19 | Payment Deferral [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases, net of unearned discount	[1]	507,491
Commercial [Member] | C O V I D19 | Other Forbearances [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases, net of unearned discount	[2]	361,664
Other Commercial Construction [Member] | C O V I D19
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases, net of unearned discount		173,353
Other Commercial Construction [Member] | C O V I D19 | Payment Deferral [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases, net of unearned discount	[1]	72,452
Other Commercial Construction [Member] | C O V I D19 | Other Forbearances [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases, net of unearned discount	[2]	100,901
Residential [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases, net of unearned discount		39,674	12,224
Residential [Member] | Payment Deferral [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases, net of unearned discount		38,557	3,247
Residential [Member] | C O V I D19
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases, net of unearned discount		3,017,626
Residential [Member] | C O V I D19 | Payment Deferral [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases, net of unearned discount	[1],[3]	3,017,626
Residential Limited Documentation [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases, net of unearned discount		1,059	3,899
Residential Limited Documentation [Member] | Payment Deferral [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases, net of unearned discount		1,059	2,667
Residential Limited Documentation [Member] | C O V I D19
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases, net of unearned discount		296,987
Residential Limited Documentation [Member] | C O V I D19 | Payment Deferral [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases, net of unearned discount	[1]	296,987
Home Equity Lines and Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases, net of unearned discount		1,660	746
Home Equity Lines and Loans [Member] | Payment Deferral [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases, net of unearned discount		1,486	412
Home Equity Lines and Loans [Member] | C O V I D19
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases, net of unearned discount		18,529
Home Equity Lines and Loans [Member] | C O V I D19 | Payment Deferral [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases, net of unearned discount	[1]	18,529
Recreational Finance [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases, net of unearned discount		2,212	90
Recreational Finance [Member] | Payment Deferral [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases, net of unearned discount		2,212	90
Recreational Finance [Member] | C O V I D19
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases, net of unearned discount		14,161
Recreational Finance [Member] | C O V I D19 | Payment Deferral [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases, net of unearned discount	[1]	14,161
Automobile [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases, net of unearned discount		4,969	182
Automobile [Member] | Payment Deferral [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases, net of unearned discount		4,955	$ 182
Automobile [Member] | C O V I D19
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases, net of unearned discount		24,882
Automobile [Member] | C O V I D19 | Payment Deferral [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases, net of unearned discount	[1]	24,882
Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases, net of unearned discount		1,434
Other [Member] | Payment Deferral [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases, net of unearned discount		1,434
Other [Member] | C O V I D19
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases, net of unearned discount		994
Other [Member] | C O V I D19 | Payment Deferral [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases, net of unearned discount	[1]	$ 994

[1] Represents accruing loans at March 31, 2021 for which a COVID-19 related payment deferral (including maturity extensions) has been granted

[2] Consists predominantly of accruing loans for which a COVID-19 related covenant waiver has been granted

[3] Includes $2.4 billion of government-guaranteed loans