Carver Bancorp Inc.-Parent Company Only Parent Company Only-Condensed Statements of Operations (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Condensed Income Statements, Captions [Line Items]
Interest expense on borrowings									$ 1,174	$ 1,257
Salaries and employee benefits									12,615	12,462
Other									7,660	8,985
Net income (loss)	$ 7,533	$ (944)	$ (594)	$ (641)	$ (2,034)	$ (893)	$ (251)	$ 325	5,354	(2,853)	$ (2,853)
Comprehensive loss									4,568	(4,486)
Parent Company
Condensed Income Statements, Captions [Line Items]
Equity in net loss from subsidiaries									6,097	(2,048)
Other income									20	26
Total income									6,117	(2,022)
Interest expense on borrowings									624	568
Shareholder expense									49	82
Other									90	181
Total expense									763	831
Net income (loss)									$ 5,354	$ (2,853)