Etratech Acquisition - Schedule of Intangible Assets (Detail) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Finite And Indefinite Lived Intangible Assets [Line Items]
Gross Value	$ 160,908	$ 111,522
Accumulated Amortization	77,622	53,965
Total other intangible assets Net	83,286	57,557
Customer relationships
Finite And Indefinite Lived Intangible Assets [Line Items]
Gross Value	101,213	66,542
Technology
Finite And Indefinite Lived Intangible Assets [Line Items]
Gross Value	47,641	$ 35,378
Etratech Inc.
Finite And Indefinite Lived Intangible Assets [Line Items]
Gross Value	33,362
Accumulated Amortization	635
Total other intangible assets Net	32,727
Etratech Inc. \| Customer relationships
Finite And Indefinite Lived Intangible Assets [Line Items]
Gross Value	24,774
Accumulated Amortization	358
Total other intangible assets Net	$ 24,416
Etratech Inc. \| Customer relationships \| Minimum
Finite And Indefinite Lived Intangible Assets [Line Items]
Estimated useful life of intangible assets	8 years
Etratech Inc. \| Customer relationships \| Maximum
Finite And Indefinite Lived Intangible Assets [Line Items]
Estimated useful life of intangible assets	12 years
Etratech Inc. \| Technology
Finite And Indefinite Lived Intangible Assets [Line Items]
Gross Value	$ 8,588
Accumulated Amortization	277
Total other intangible assets Net	$ 8,311
Etratech Inc. \| Technology \| Minimum
Finite And Indefinite Lived Intangible Assets [Line Items]
Estimated useful life of intangible assets	5 years
Etratech Inc. \| Technology \| Maximum
Finite And Indefinite Lived Intangible Assets [Line Items]
Estimated useful life of intangible assets	6 years

Finite and indefinite lived intangible assets.

No definition available.

Name:	thrm_FiniteAndIndefiniteLivedIntangibleAssetsLineItems
Namespace Prefix:	thrm_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Useful life of finite-lived intangible assets, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

Name:	us-gaap_FiniteLivedIntangibleAssetUsefulLife
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Accumulated amount of amortization of assets, excluding financial assets and goodwill, lacking physical substance with a finite life.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 30
-Section 50
-Paragraph 2
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=66006027&loc=d3e16323-109275

+ Details

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

- Definition

Amount before amortization of assets, excluding financial assets and goodwill, lacking physical substance with a finite life.

+ References

+ Details

Name:	us-gaap_FiniteLivedIntangibleAssetsGross
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount after amortization of assets, excluding financial assets and goodwill, lacking physical substance with a finite life.

+ References

+ Details

Name:	us-gaap_FiniteLivedIntangibleAssetsNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis=us-gaap_CustomerRelationshipsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis=us-gaap_TechnologyBasedIntangibleAssetsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_BusinessAcquisitionAxis=thrm_EtratechIncMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RangeAxis=us-gaap_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RangeAxis=us-gaap_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: