Loans and Leases (Narrative) (Detail) $ in Thousands	3 Months Ended
	Mar. 31, 2022 USD ($) loan	Mar. 31, 2021 USD ($)	Jan. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Increase in loans and leases	$ 21,300,000
Loans and leases purchase discount	317,600
Unamortized premiums	(124,100)			$ 12,300
Interest receivable	122,600			50,700
Interest on non-accrual loans	$ 4,400	$ 4,000
Loans and leases modified as TDRs within previous 12 months for which there was a payment default | loan	0
Sterling
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans and leases			$ 20,498,706
Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Write-down	$ 9,100	1,600
Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Write-down	100	$ 300
Collateral Pledged | FHLB
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans pledged as collateral	7,600,000
Collateral Pledged | FRB
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans pledged as collateral	200,000
Collateral Pledged | Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Impaired financing receivable	102,200			16,600
Collateral Pledged | Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Impaired financing receivable	32,100			34,900
Collateral Value
Accounts, Notes, Loans and Financing Receivable [Line Items]
Impaired financing receivable	$ 139,900			$ 86,000