Loans Receivable and Allowance for Loan Losses - Additional Information (Detail)		12 Months Ended
	Jan. 01, 2020 USD ($)	Dec. 31, 2020 USD ($) SecurityLoan counterparty Contract borrower	Dec. 31, 2019 USD ($) borrower Contract SecurityLoan	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Less allowance for credit losses		$ 101,466,000	$ 55,525,000	$ 55,562,000	$ 60,195,000
Retained earnings		718,090,000	695,273,000
Decreased interest income		1,000,000.0	845,000	894,000
Principal amount of nonaccrual loans		$ 87,100,000	$ 40,200,000
Number of loans 90 days past due and still accruing | Contract		0	0
Increase in interest income		$ 3,200,000	$ 1,700,000	1,400,000
Interest income, cash basis		1,875,000	$ 2,123,000	$ 2,000,000.0
Impaired loan defined floor limit (greater than)		$ 1,000,000.0
Impaired loans number | SecurityLoan		169	158
Impaired loans		$ 86,046,000	$ 70,641,000
Number of borrowers | borrower		110	128
Collateral-dependent impaired loans		$ 26,300,000	$ 20,400,000
Credit loss expense for off-balance sheet credit exposures		29,700,000
Allowances for loan losses		$ 8,974,000	$ 5,067,000
Yield percentage rate		5.44%	3.82%
Weighted average modified interest rate		5.43%	3.83%
Number of payment defaults for loans modified as TDRs | SecurityLoan		0	1
Purchased credit-impaired loans		$ 0	$ 746,000
Average balances of impaired loans		66,800,000	74,100,000
Loan commitments		1,990,000,000	1,260,000,000
Undisbursed home equity and personal credit lines		241,200,000	212,400,000
Loans		$ 9,822,890,000	$ 7,332,885,000
Number of PPP loans | counterparty		1,287
Paycheck protection program		$ 473,200,000
SB One Bancorp
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Less allowance for credit losses		$ 13,600,000
Impaired Loans Troubled Debt Restructurings
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired loans number | SecurityLoan		135	147
Impaired loans		$ 39,600,000	$ 48,300,000
Number of troubled debt restructurings | SecurityLoan		112	133
Loans and leases receivable, impaired, nonperforming, accrual of interest		$ 23,100,000	$ 42,700,000
Charge-offs		7,300,000	11,600,000
Allowances for loan losses		362,000	177,130
No Allowance For Loan Losses Required
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired financing receivable with no related allowance		43,700,000	$ 16,000,000.0
Consumer Loan
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Collateral-dependent impaired loans		$ 9,000
Commercial
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired loans number | SecurityLoan		169	158
Impaired loans		$ 86,000,000.0	$ 70,600,000
Commercial | Non Accrual
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired loans number | SecurityLoan		55	25
Impaired loans		$ 61,400,000	$ 27,900,000
Commercial Loan
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Collateral-dependent impaired loans		12,800,000
Residential Real Estate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Collateral-dependent impaired loans		$ 13,400,000
Accounting Standards Update 2016-13
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Less allowance for credit losses	$ 7,900,000
Retained earnings	8,300,000
Credit loss expense for off-balance sheet credit exposures	3,200,000
Accounting Standards Update 2016-13, Effect On Loans Receivable
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Retained earnings	$ 5,900,000