Debt instrument maturity month and year range end.

No definition available.

Name:	thrm_DebtInstrumentMaturityMonthAndYearRangeEnd
Namespace Prefix:	thrm_
Data Type:	xbrli:gYearMonthItemType
Balance Type:	na
Period Type:	duration

- Definition

Debt instrument maturity month and year range start.

+ References

No definition available.

+ Details

Name:	thrm_DebtInstrumentMaturityMonthAndYearRangeStart
Namespace Prefix:	thrm_
Data Type:	xbrli:gYearMonthItemType
Balance Type:	na
Period Type:	duration

- Definition

Line of credit facility increase in maximum borrowing capacity.

+ References

No definition available.

+ Details

Name:	thrm_LineOfCreditFacilityIncreaseInMaximumBorrowingCapacity
Namespace Prefix:	thrm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Line of credit facility increase maximum borrowings subject to specific conditions.

+ References

No definition available.

+ Details

Name:	thrm_LineOfCreditFacilityIncreaseMaximumBorrowingsSubjectToSpecificConditions
Namespace Prefix:	thrm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Percentage of stock interest on subsidiaries pledge to secure obligation.

+ References

No definition available.

+ Details

Name:	thrm_PercentageOfStockInterestOnSubsidiariesPledgeToSecureObligation
Namespace Prefix:	thrm_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Date when the debt instrument is scheduled to be fully repaid, in CCYY-MM-DD format.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(2))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bbb)(2)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

+ Details

Name:	us-gaap_DebtInstrumentMaturityDate
Namespace Prefix:	us-gaap_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Definition

The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(16))
-URI http://asc.fasb.org/extlink&oid=119400593&loc=d3e572229-122910

+ Details

Name:	us-gaap_LineOfCredit
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The effective interest rate during the reporting period.

+ References

+ Details

Name:	us-gaap_LineOfCreditFacilityInterestRateDuringPeriod
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ References

+ Details

Name:	us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of borrowing capacity currently available under the credit facility (current borrowing capacity less the amount of borrowings outstanding).

+ References

+ Details

Name:	us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

Name:	us-gaap_VariableRateAxis=us-gaap_FederalFundsEffectiveSwapRateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VariableRateAxis=us-gaap_LondonInterbankOfferedRateLIBORMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=thrm_UnitedStateBankOfAmericaCreditFacilityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=thrm_EuroCurrencyRateLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=thrm_CreditAgreementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=thrm_BaseRateLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=thrm_DEGVietnamLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CreditFacilityAxis=us-gaap_RevolvingCreditFacilityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=thrm_UnitedStateRevolvingNoteMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=thrm_LettersOfCreditMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=thrm_SwingLineLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Debt - Additional Information (Detail) - USD ($)			3 Months Ended
Debt - Additional Information (Detail) - USD ($)	Jun. 27, 2019	Jun. 26, 2019	Mar. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Debt Instrument [Line Items]
Maximum percentage of stock of non US subsidiaries pledge to secure obligation			66.00%
Federal Funds Rate \| Minimum
Debt Instrument [Line Items]
Interest rate			0.50%
Federal Funds Rate \| Maximum
Debt Instrument [Line Items]
Interest rate			0.08%
London Interbank Offered Rate
Debt Instrument [Line Items]
Interest rate			0.99%
United State Bank Of America Credit Facility
Debt Instrument [Line Items]
Interest rate			3.25%
Euro Currency Rate Loans
Debt Instrument [Line Items]
Interest rate			1.00%
Euro Currency Rate Loans \| Credit Agreement \| Minimum
Debt Instrument [Line Items]
Interest rate			1.25%
Euro Currency Rate Loans \| Credit Agreement \| Maximum
Debt Instrument [Line Items]
Interest rate			2.25%
Base Rate Loans \| Credit Agreement \| Minimum
Debt Instrument [Line Items]
Interest rate			0.25%
Base Rate Loans \| Credit Agreement \| Maximum
Debt Instrument [Line Items]
Interest rate			1.25%
DEG Vietnam Loan
Debt Instrument [Line Items]
Semi-annual principal payments earliest date			2017-11
Semi-annual principal payments latest date			2023-05
Revolving Credit Facility \| Revolving Note (U.S. Dollar)
Debt Instrument [Line Items]
Maximum borrowing capacity	$ 475,000,000	$ 350,000,000			$ 350,000,000
Maximum borrowing capacity increase subject to specified conditions			$ 175,000,000
Line of credit, outstanding amount			225,417,000
Debt maturity date	Jun. 27, 2024	Mar. 17, 2021
Increase in maximum borrowing capacity			169,546,000
Remaining borrowing capacity			226,743,000
Revolving Credit Facility \| Revolving Note (U.S. Dollar) \| Letters of Credit
Debt Instrument [Line Items]
Maximum borrowing capacity			15,000,000
Line of credit, outstanding amount			0	$ 0
Revolving Credit Facility \| Revolving Note (U.S. Dollar) \| Swing Line Loans
Debt Instrument [Line Items]
Maximum borrowing capacity			$ 40,000,000