Loans - Non-Accrual at Amortized Cost (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2021	Dec. 31, 2020	Dec. 31, 2019
Total Non-accrual amortized cost	$ 18,611	$ 20,947	$ 28,026
Non-Accrual with no related Allowance	16,986	16,880
Interest income recognized	87	58
Loans ninety days or more past due and still accruing:	1,925	2,748
Multi-family Residential Portfolio Segment [Member]
Total Non-accrual amortized cost	4,461	2,576	2,723
Non-Accrual with no related Allowance	4,461	2,576
Interest income recognized	19
Loans ninety days or more past due and still accruing:		201
Commercial Real Estate
Total Non-accrual amortized cost	640	1,766	2,714
Non-Accrual with no related Allowance	640	1,766
Loans ninety days or more past due and still accruing:		2,547
One-To-Four Family - Mixed-Use Property Portfolio Segment [Member]
Total Non-accrual amortized cost	2,510	1,706	1,704
Non-Accrual with no related Allowance	2,510	1,706
Interest income recognized	4
One-To-Four Family - Residential Portfolio Segment [Member]
Total Non-accrual amortized cost	7,509	5,313	9,992
Non-Accrual with no related Allowance	7,509	5,313
Interest income recognized	1
Construction Portfolio Segment [Member]
Loans ninety days or more past due and still accruing:	873
Small Business Administration Portfolio Segment [Member]
Total Non-accrual amortized cost	991	1,168	1,169
Non-Accrual with no related Allowance	991	1,168
Taxi Medallion Portfolio Segment [Member]
Total Non-accrual amortized cost		2,758	2,318
Non-Accrual with no related Allowance		2,758
Commercial Business and Other Portfolio Segment [Member]
Total Non-accrual amortized cost	2,500	5,660	$ 7,406
Non-Accrual with no related Allowance	875	1,593
Interest income recognized	63	$ 58
Loans ninety days or more past due and still accruing:	$ 1,052