Carver Bancorp Inc.-Parent Company Only Parent Company Only-Condensed Statements of Cash Flow (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 Cash Flow Statements, Captions [Line Items]
Net income (loss)	$ 7,533	$ (944)	$ (594)	$ (641)	$ (2,034)	$ (893)	$ (251)	$ 325	$ 5,354	$ (2,853)	$ (2,853)
(Increase) decrease in other assets									(1,173)	1,641
Decrease in other liabilities									(870)	(147)
Net cash (used in) provided by operating activities									(4,985)	827
Net decrease in cash									75,872	(4,502)
Parent Company
Condensed Cash Flow Statements, Captions [Line Items]
Net income (loss)									5,354	(2,853)
Equity in net loss from subsidiaries									6,097	(2,048)
(Increase) decrease in other assets									(51)	54
Increase (decrease) in accounts payable to subsidiaries									166	176
Decrease in other liabilities									630	(1,928)
Net cash (used in) provided by operating activities									2	(2,503)
Net decrease in cash									2	(2,503)
Cash and cash equivalents - beginning				$ 492				$ 2,995	492	2,995
Cash and cash equivalents - end	$ 494				$ 492				$ 494	$ 492	$ 2,995