Carver Bancorp Inc.- Parent Company Only-Condensed Statements of Operations (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Mar. 31, 2021	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2021	Mar. 31, 2020
Condensed Income Statements, Captions [Line Items]
Grant income									$ 500	$ 0
Interest expense on borrowings									650	991
Salaries and employee benefits									11,180	11,372
Shareholder expense									366	331
Other									5,394	5,606
Net loss	$ (965)	$ (1,310)	$ (809)	$ (812)	$ (1,798)	$ (1,436)	$ (1,050)	$ (1,139)	(3,896)	(5,423)
Comprehensive loss									(8,003)	(3,552)
Parent Company
Condensed Income Statements, Captions [Line Items]
Equity in net loss from subsidiaries									(3,039)	(4,499)
Other income									21	26
Grant income									500	0
Total income									(2,518)	(4,473)
Interest expense on borrowings									562	815
Salaries and employee benefits									168	0
Shareholder expense									223	32
Other									425	103
Total expense									1,378	950
Net loss									$ (3,896)	$ (5,423)