Quarterly Financial Data (Unaudited) (Details) - USD ($) $ in Thousands	3 Months Ended								6 Months Ended	9 Months Ended	12 Months Ended
	Dec. 31, 2023	Sep. 30, 2023	Jun. 30, 2023	Mar. 31, 2023	Dec. 31, 2022	Sep. 30, 2022	Jun. 30, 2022	Mar. 31, 2022	Jun. 30, 2023	Sep. 30, 2023	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Net Income (Loss)	$ 8,099	$ 7,835	$ 8,686	$ 4,044	$ 10,249	$ 23,442	$ 25,035	$ 18,219			$ 28,664	$ 76,945	$ 81,793
Error Correction, Type [Extensible Enumeration]		ffic:AccountingTreatmentOfEmployeeRetentionCreditsMember	ffic:AccountingTreatmentOfEmployeeRetentionCreditsMember	ffic:AccountingTreatmentOfEmployeeRetentionCreditsMember					ffic:AccountingTreatmentOfEmployeeRetentionCreditsMember	ffic:AccountingTreatmentOfEmployeeRetentionCreditsMember
Error correction adjustment
Net Income (Loss)		$ (1,600)	$ 100	$ (1,100)					$ (1,100)	$ (2,600)