Investment in Aizu Fujitsu Semiconductor Wafer Solution Limited ("AFSW") - Summarized Financial Information (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020	Dec. 31, 2019
Variable Interest Entity [Line Items]
Current assets	$ 14,294		$ 18,226	$ 5,357
Other current liabilities	1,866		1,466	1,688
Aizu Fajitsu Semiconductor Wafer Solution Limited
Variable Interest Entity [Line Items]
Current assets	932		1,503	3,733
Long-term assets	5,330		5,572	5,101
Other current liabilities	2,200		2,521	931
Net deficit	(31,471)		(30,549)	(15,359)
Variable Interest Entity, Measure of Activity [Abstract]
Sales	842	$ 1,248	2,976	11,599
Gross loss	(2,425)	(2,197)	(11,411)	(4,849)
Net loss	(2,995)	$ (2,895)	(13,952)	(7,557)
Fujitsu Semiconductor Limited | Aizu Fajitsu Semiconductor Wafer Solution Limited
Variable Interest Entity [Line Items]
Accounts payable due to related parties	22,354		22,136	17,913
Transphorm, Inc. | Aizu Fajitsu Semiconductor Wafer Solution Limited
Variable Interest Entity [Line Items]
Accounts payable due to related parties	$ 13,179		$ 12,967	$ 5,349