Actuarial Assumptions Used to Determine Benefit Obligation and Net Periodic Service Cost (Detail)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Discount rate	3.65%	2.95%	3.25%
Gentherm GmbH
Defined Benefit Plan Disclosure [Line Items]
Discount rate	2.04%	1.93%	1.69%
Expected long term rate of return on plan assets	3.40%	3.40%	3.40%