Loans Receivable and Allowance for Credit Losses (Narrative) (Details)	3 Months Ended		12 Months Ended
	Mar. 31, 2021 USD ($) borrower SecurityLoan counterparty troubled_debt_restructuring	Mar. 31, 2020 USD ($)	Dec. 31, 2020 USD ($) SecurityLoan borrower troubled_debt_restructuring	Jul. 31, 2020 USD ($)	Jan. 01, 2020 USD ($)	Dec. 31, 2019 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total gross loans	$ 9,819,643,000		$ 9,839,858,000
Less allowance for credit losses	85,591,000	$ 75,143,000	101,466,000		$ 55,525,000	$ 55,525,000
Retained earnings	748,574,000		718,090,000
Principal amount of nonaccrual loans	82,100,000		87,100,000
Loans less than 90 days past due	$ 27,100,000		$ 35,300,000
Number of loans 90 days past due and still accruing | SecurityLoan	0		0
Impaired loan defined floor limit (greater than)	$ 1,000,000.0
Impaired loans number | SecurityLoan	167		169
Impaired loans	$ 79,547,000		$ 86,046,000
Number of troubled debt restructurings | troubled_debt_restructuring	115		135
Number of borrowers | borrower	112		110
Loans and leases receivable, impaired, nonperforming, accrual of interest			$ 39,600,000
Collateral-dependent impaired loans	$ 30,100,000		26,300,000
Provision (benefit) charge for credit losses	(15,001,000)	14,717,000
Charge off on TDRs	$ 1,500,000
Number of payment defaults for loans modified as TDRs | SecurityLoan	0
Purchased credit-impaired loans			746,000
Allowances for loan losses	$ 4,802,000		8,974,000
Impaired financing receivable with no related allowance	50,300,000		43,700,000
Average balance of impaired loans	86,426,000		66,763,000
Total gross loans	$ 9,803,536,000		9,822,890,000
Number of PPP loans | counterparty	1,318
Paycheck protection program	$ 664,500,000
Number of PPP loans forgiven | SecurityLoan	670
Paycheck protection program, amount forgiven	$ 182,600,000
Paycheck protection program, amount outstanding	481,900,000
SB One Bancorp
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Less allowance for credit losses				$ 13,600,000
Purchased credit-impaired (PCI) loans | SB One Bancorp
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Less allowance for credit losses	10,800,000		13,100,000
Total gross loans	283,900,000		296,600,000
Performing Financial Instruments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans and leases receivable, impaired, nonperforming, accrual of interest	22,700,000		$ 23,100,000
Consumer Loan
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Collateral-dependent impaired loans	216,000
Commercial Loan
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Collateral-dependent impaired loans	28,300,000
Residential Real Estate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Collateral-dependent impaired loans	1,600,000
Impaired Loans Troubled Debt Restructurings
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired loans number | SecurityLoan			112
Allowances for loan losses	0
Commercial Loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total gross loans	2,510,708,000		$ 2,567,470,000
Less allowance for credit losses	26,302,000	$ 22,841,000	27,084,000		28,263,000	$ 28,263,000
Impaired loans	28,359,000		35,832,000
Allowances for loan losses	3,832,000		4,715,000
Average balance of impaired loans	32,533,000		$ 31,411,000
Commercial Loans | SB One Bancorp
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total gross loans	$ 101,700,000
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
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