Summary of Significant Accounting Policies (Details) shares in Thousands, $ in Thousands		3 Months Ended								12 Months Ended
	Oct. 30, 2020 USD ($)	Dec. 31, 2020 USD ($) item	Sep. 30, 2020 USD ($)	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2020 USD ($) item shares	Dec. 31, 2019 USD ($) shares	Dec. 31, 2018 USD ($) shares
Cash and Cash Equivalents, at Carrying Value, Ending Balance		$ 157,400				$ 49,800				$ 157,400	$ 49,800
Restricted Cash and Cash Equivalents		$ 63,500								$ 63,500
Restricted Cash and Cash Equivalents, Asset, Statement of Financial Position [Extensible List]		Cash and Cash Equivalents, at Carrying Value, Ending Balance								Cash and Cash Equivalents, at Carrying Value, Ending Balance
Interest-bearing Deposits in Banks and Other Financial Institutions		$ 133,700				36,500				$ 133,700	36,500
Cash Reserve Deposit Required and Made						14,400					14,400
Interest Income, Securities, Operating, Tax Exempt										1,900	2,000	$ 3,400
Goodwill, Impairment Loss										0	0	0
Goodwill, Ending Balance		17,636				16,127				17,636	16,127
Financing Receivable Purchase										193,300	221,200	294,600
Financing Receivable Sale										7,400	13,700	14,000
Provision for credit loan losses		3,862	$ 2,470	$ 9,619	$ 7,178	(318)	$ 683	$ 1,474	$ 972	22,600	2,811	575
Assets Acquired	$ 286,100
Allowance for Loans Losses	$ 4,100
Loans Receivable Held-for-sale, Net, Not Part of Disposal Group, Ending Balance		0				0				0	0
Other than Temporary Impairment Losses, Investments, Total										0
Advertising Expense										$ 1,800	$ 2,200	$ 2,200
Number of business units | item										3
Flushing Financial Capital Trust II [Member]
Capital Securities, Value Issued										$ 60,000
Common Securities With Voting Rights, Value , Issued										$ 1,900
Percentage Of Ownership Interest In Common Securities Of Trust										100.00%
Share-based Payment Arrangement, Option
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount | shares										0	0	0
Bank Premises and Equipment [Member] | Minimum [Member]
Property, Plant and Equipment, Useful Life										3 years
Bank Premises and Equipment [Member] | Maximum [Member]
Property, Plant and Equipment, Useful Life										10 years
Consumer Portfolio Segment [Member]
Mortgage Loans in Process of Foreclosure, Amount		$ 5,900				6,600				$ 5,900	$ 6,600
Loans Receivable [Member]
Number of active forbearances | item		134								134
Outstanding balance in active forbearance		$ 364,400								$ 364,400
Financing Receivable		23,600								23,600
Interest Receivable		$ 41,500				$ 19,700				$ 41,500	$ 19,700