Income Taxes (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020	Dec. 31, 2019
Operating Loss Carryforwards [Line Items]
Loss before tax expense	$ 6,579	$ 4,187	$ 17,909	$ 15,283
Tax expense	0	$ 0	0	0
Valuation allowance	53,500		52,200	49,400
Domestic Tax Authority
Operating Loss Carryforwards [Line Items]
Loss before tax expense	4,900		10,300	10,600
Domestic Tax Authority
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards	250,000
Operating loss carryforwards subject to expiration	207,500
Domestic Tax Authority | Tax Year 2019
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards	8,800
Domestic Tax Authority | Tax Year 2020
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards	10,500
Domestic Tax Authority | Tax Year 2021
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards	4,500
Domestic Tax Authority | Research Tax Credit Carryforward
Operating Loss Carryforwards [Line Items]
Research tax credit carryforward	4,600
Foreign Tax Authority
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards	5,400
Foreign Tax Authority | National Tax Agency, Japan
Operating Loss Carryforwards [Line Items]
Loss before tax expense	1,700		7,600	4,700
Foreign Tax Authority | Transphorm Japan, Inc. | National Tax Agency, Japan
Operating Loss Carryforwards [Line Items]
Loss before tax expense	1,500		800	1,000
Foreign Tax Authority | Transphorm Ailzu, Inc. | National Tax Agency, Japan
Operating Loss Carryforwards [Line Items]
Loss before tax expense			6,800	3,700
Foreign Tax Authority | Transphorm Epi, Inc. | National Tax Agency, Japan
Operating Loss Carryforwards [Line Items]
Loss before tax expense			(25)	(1)
State and Local Jurisdiction
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards	153,500
Tax expense			$ 1	$ 1
State and Local Jurisdiction | Tax Year 2018
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards	18,500
State and Local Jurisdiction | Tax Year 2019
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards	2,300
State and Local Jurisdiction | Tax Year 2020
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards	2,600
State and Local Jurisdiction | Tax Year 2021
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards	1,000
State and Local Jurisdiction | Research Tax Credit Carryforward
Operating Loss Carryforwards [Line Items]
Research tax credit carryforward	$ 4,200