Pension and Other Post Retirement Benefit Plans - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2017 USD ($) Payment	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, amendment date	Dec. 04, 2017
Discount rate for net periodic service cost	3.25%	3.40%	3.25%
Discount rate for benefit obligation	2.95%	3.25%	3.40%
Insurance policy value	$ 2,353,000	$ 2,112,000
Tax (expense) benefit recognized in other comprehensive income	26,000	14,000	$ (23,000)
Pretax unrecognized actuarial losses recorded in accumulated other comprehensive income (loss)	380,000	302,000
Expected pretax unrecognized actuarial losses recorded in accumulated other comprehensive income (loss ) in next fiscal year	0
Contributions to 401(k) plan	$ 1,396,000	$ 1,289,000	$ 959,000
Gentherm GmbH
Defined Benefit Plan Disclosure [Line Items]
Discount rate for benefit obligation	1.93%	1.69%	2.21%
Tax (expense) benefit recognized in other comprehensive income	$ (86,000)	$ 171,000	$ (211,000)
Pretax unrecognized actuarial losses recorded in accumulated other comprehensive income (loss)	2,416,000	2,406,000
Expected pretax unrecognized actuarial losses recorded in accumulated other comprehensive income (loss ) in next fiscal year	74,000
Pension benefit obligation	4,036,000	4,000,000
Underfunded defined benefit plan	4,036,000	$ 4,000,000
Defined benefit plan, expected employer contribution in next fiscal year	0
Former Chief Executive Officer
Defined Benefit Plan Disclosure [Line Items]
Annual retirement payment	$ 300,000,000
Number of expected annual retirement benefit payments | Payment	15
Annual retirement benefit payments, earliest date	Jan. 01, 2018
Former Chief Executive Officer | Plan Amendment
Defined Benefit Plan Disclosure [Line Items]
Expected annual retirement payments	$ 342,000,000
Defined benefit plan amendment description	On May 10, 2017 the Company amended (the “Plan Amendment”) the Plan. Prior to the Plan Amendment, the Plan provided for 15 annual retirement benefit payments of $300,000 each beginning January 1, 2018. Mr. Coker became fully vested in the benefits under the Benefit Plan on April 1, 2017. The Plan Amendment provided that if Mr. Coker continued to provide employment service to the Company through and including January 1, 2018, the fifteen annual retirement benefit payments would be increased to $342,000, otherwise the 15 annual retirement benefits would remain at $300,000.
Former Chief Executive Officer | Retirement Agreement
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan amendment description	On June 28, 2017, the same day a leader transition plan leading up to Mr. Coker’s retirement was announced, the Company entered into a Retirement Agreement with Mr. Coker (the “Retirement Agreement”). The Retirement Agreement provided that if Mr. Coker’s retirement date was prior to January 1, 2018, the Company would amend the terms of the Plan, as amended, to accelerate the in-service vesting date to ensure an increase in the annual accrued benefit from $300,000 to $342,000. Mr. Coker became fully vested in the incremental benefit, as described in the Plan Amendment, on December 4, 2017.
Annual accrued benefit payments	$ 300,000,000
Expected annual accrued benefit payments	$ 342,000,000