Carver Bancorp Inc.-Parent Company Only Parent Company Only-Condensed Statements of Financial Condition (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Mar. 31, 2017	Sep. 30, 2016	Mar. 31, 2016
Condensed Balance Sheet Statements, Captions [Line Items]
Other assets	$ 7,180	$ 5,829
Total assets	693,910	687,861
Other liabilities	16,653	11,884
Total liabilities	641,939	640,463
Stockholders’ equity	51,971	47,398	$ 51,880
Total liabilities and equity	693,910	687,861
Parent Company
Condensed Balance Sheet Statements, Captions [Line Items]
Cash on deposit with subsidiaries	494	492		$ 2,995
Investment in subsidiaries	66,235	60,921
Other assets	66	15
Total assets	66,795	61,428
Borrowings	13,403	13,403
Accounts payable to subsidiaries	393	228
Other liabilities	1,028	399
Total liabilities	14,824	14,030
Stockholders’ equity	51,971	47,398
Total liabilities and equity	$ 66,795	$ 61,428