Commitments and Contingencies - Summary of Insurance Recoveries for Asbestos Related Liabilities (Details) - Bendix $ in Millions	12 Months Ended
Dec. 31, 2018 USD ($)
Loss Contingencies [Line Items]
Beginning of year	$ 191
Probable insurance recoveries related to estimated liability	10
Insurance receipts for asbestos-related liabilities	(24)
Insurance receivables settlements and write-offs	1
Spin-Off related adjustments	$ (178)

Loss contingency insurance receivables settlements.

No definition available.

Name:	gtx_LossContingencyInsuranceReceivablesSettlements
Namespace Prefix:	gtx_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Loss contingency receivables spin-off related adjustment.

+ References

No definition available.

+ Details

Name:	gtx_LossContingencyReceivablesSpinOffRelatedAdjustment
Namespace Prefix:	gtx_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

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

- Definition

Amount of receivable related to a loss contingency accrual. For example, an insurance recovery receivable.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 460
-SubTopic 10
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=121555522&loc=d3e12053-110248

+ Details

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

- Definition

Amount of cash inflow from the collection of receivables related to a loss contingency.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 460
-SubTopic 10
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=121555522&loc=d3e12053-110248

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 17
-Subparagraph (f)
-URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3367-108585

+ Details

Name:	us-gaap_LossContingencyReceivableProceeds
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of additions to receivables pertaining to a loss contingency.

+ References

+ Details

Name:	us-gaap_LossContingencyRelatedReceivableCarryingValueAdditions
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	us-gaap_LossContingenciesByNatureOfContingencyAxis=gtx_BendixAsbestosRelatedLiabilitiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: