Investment in Aizu Fujitsu Semiconductor Wafer Solution Limited ("AFSW") - Summarized financial information (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2021	Dec. 31, 2020	Mar. 31, 2021
Variable Interest Entity [Line Items]
Current assets	$ 50,661		$ 50,661		$ 14,294
Other current liabilities	0		0		1,866
Aizu Fajitsu Semiconductor Wafer Solution Limited
Variable Interest Entity [Line Items]
Current assets	1,844		1,844		932
Long-term assets	5,152		5,152		5,330
Other current liabilities	3,494		3,494		2,200
Net deficit	(799)		(799)		(31,471)
Variable Interest Entity, Measure of Activity [Abstract]
Sales	2,262	$ 698	5,957	$ 1,728
Gross loss	(2,338)	(2,896)	(7,094)	(9,214)
Net loss	(2,848)	$ (3,303)	(9,085)	$ (11,057)
Fujitsu Semiconductor Limited | Aizu Fajitsu Semiconductor Wafer Solution Limited
Variable Interest Entity [Line Items]
Due to controlling owner	3,707		3,707		22,354
Transphorm, Inc. | Aizu Fajitsu Semiconductor Wafer Solution Limited
Variable Interest Entity [Line Items]
Due to Transphorm	$ 594		$ 594		$ 13,179