Goodwill and Intangible Assets - Changes in Carrying Amount of Goodwill (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017
Changes in the carrying amount of goodwill by segment
Balance, beginning	$ 4,752	$ 4,558
Acquisitions & divestitures		1
Foreign currency translation	(119)	193
Balance, ending	4,633	4,752
Cumulative goodwill impairment charges, December 31, 2018	240
Automotive OEM
Changes in the carrying amount of goodwill by segment
Balance, beginning	488	456
Acquisitions & divestitures		0
Foreign currency translation	(12)	32
Balance, ending	476	488
Cumulative goodwill impairment charges, December 31, 2018	24
Test & Measurement and Electronics
Changes in the carrying amount of goodwill by segment
Balance, beginning	1,372	1,336
Acquisitions & divestitures		0
Foreign currency translation	(20)	36
Balance, ending	1,352	1,372
Cumulative goodwill impairment charges, December 31, 2018	83
Food Equipment
Changes in the carrying amount of goodwill by segment
Balance, beginning	269	249
Acquisitions & divestitures		0
Foreign currency translation	(10)	20
Balance, ending	259	269
Cumulative goodwill impairment charges, December 31, 2018	60
Polymers & Fluids
Changes in the carrying amount of goodwill by segment
Balance, beginning	919	889
Acquisitions & divestitures		0
Foreign currency translation	(30)	30
Balance, ending	889	919
Cumulative goodwill impairment charges, December 31, 2018	15
Welding
Changes in the carrying amount of goodwill by segment
Balance, beginning	272	260
Acquisitions & divestitures		0
Foreign currency translation	(9)	12
Balance, ending	263	272
Cumulative goodwill impairment charges, December 31, 2018	5
Construction Products
Changes in the carrying amount of goodwill by segment
Balance, beginning	530	508
Acquisitions & divestitures		0
Foreign currency translation	(17)	22
Balance, ending	513	530
Cumulative goodwill impairment charges, December 31, 2018	7
Specialty Products
Changes in the carrying amount of goodwill by segment
Balance, beginning	902	860
Acquisitions & divestitures		1
Foreign currency translation	(21)	41
Balance, ending	881	$ 902
Cumulative goodwill impairment charges, December 31, 2018	$ 46

Goodwill, Acquired, During Period And Written Off Related To Sale Of Business Unit

No definition available.

Name:	itw_GoodwillAcquiredDuringPeriodAndWrittenOffRelatedToSaleOfBusinessUnit
Namespace Prefix:	itw_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 45
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=77989000&loc=SL49117168-202975

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=99380562&loc=d3e13770-109266

+ Details

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

- Definition

Amount of accumulated impairment loss for an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (h)
-URI http://asc.fasb.org/extlink&oid=108376223&loc=d3e13816-109267

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=108376223&loc=d3e13816-109267

+ Details

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

- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

+ Details

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

- Definition

Amount of increase (decrease) from foreign currency translation adjustments and purchase accounting adjustments of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 10
-Section 25
-Paragraph 16
-URI http://asc.fasb.org/extlink&oid=84229725&loc=d3e961-128460

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=108376223&loc=d3e13816-109267

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (f)
-URI http://asc.fasb.org/extlink&oid=108376223&loc=d3e13816-109267

+ Details

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

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=itw_AutomotiveOEMSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=itw_TestandMeasurementandElectronicsSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=itw_FoodEquipmentSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=itw_PolymersAndFluidsSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=itw_WeldingSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=itw_ConstructionProductsSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=itw_SpecialtyProductsSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: