Cover Page	6 Months Ended
	Jun. 30, 2021 shares
Entity Information [Line Items]
Document Type	10-Q/A
Document Quarterly Report	true
Document Period End Date	Jun. 30, 2021
Document Transition Report	false
Entity File Number	1-4797
Entity Registrant Name	ILLINOIS TOOL WORKS INC
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	36-1258310
Entity Address, Address Line One	155 Harlem Avenue
Entity Address, City or Town	Glenview
Entity Address, State or Province	IL
Entity Address, Postal Zip Code	60025
City Area Code	847
Local Phone Number	724-7500
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding	314,967,814
Amendment Description	Illinois Tool Works Inc. is filing this Amendment No. 1 on Form 10-Q/A (this “Amendment”) to its Quarterly Report on Form 10-Q for the quarter ended June 30, 2021, originally filed on August 5, 2021 (the “Original 10-Q”), solely for the purpose of adding conformed signatures to the certification filed as Exhibit 32 pursuant to Rule 13(a) under the Securities Exchange Act, as amended. The Original 10-Q is being re-filed in its entirety with the certifications of the registrant’s principal executive officer and principal financial officer being made as of the date of this Amendment. The conformed signatures were inadvertently omitted in the Original 10-Q.Except as expressly set forth above, this Amendment does not modify or update disclosure in, or exhibits to, the Original 10-Q. Information not affected by this Amendment remains unchanged and reflects the disclosures made at the time the Original 10-Q was filed.
Entity Central Index Key	0000049826
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	Q2
Amendment Flag	true
NEW YORK STOCK EXCHANGE, INC. | Common Stock
Entity Information [Line Items]
Title of 12(b) Security	Common Stock
Trading Symbol	ITW
Security Exchange Name	NYSE
NEW YORK STOCK EXCHANGE, INC. | 1.75% Euro Notes due 2022
Entity Information [Line Items]
Title of 12(b) Security	1.75% Euro Notes due 2022
Trading Symbol	ITW22
Security Exchange Name	NYSE
NEW YORK STOCK EXCHANGE, INC. | 1.25% Euro Notes due 2023
Entity Information [Line Items]
Title of 12(b) Security	1.25% Euro Notes due 2023
Trading Symbol	ITW23
Security Exchange Name	NYSE
NEW YORK STOCK EXCHANGE, INC. | 0.250% Euro Notes due 2024
Entity Information [Line Items]
Title of 12(b) Security	0.250% Euro Notes due 2024
Trading Symbol	ITW24A
Security Exchange Name	NYSE
NEW YORK STOCK EXCHANGE, INC. | 0.625% Euro Notes due 2027
Entity Information [Line Items]
Title of 12(b) Security	0.625% Euro Notes due 2027
Trading Symbol	ITW27
Security Exchange Name	NYSE
NEW YORK STOCK EXCHANGE, INC. | 2.125% Euro Notes due 2030
Entity Information [Line Items]
Title of 12(b) Security	2.125% Euro Notes due 2030
Trading Symbol	ITW30
Security Exchange Name	NYSE
NEW YORK STOCK EXCHANGE, INC. | 1.00% Euro Notes due 2031
Entity Information [Line Items]
Title of 12(b) Security	1.00% Euro Notes due 2031
Trading Symbol	ITW31
Security Exchange Name	NYSE
NEW YORK STOCK EXCHANGE, INC. | 3.00% Euro Notes due 2034
Entity Information [Line Items]
Title of 12(b) Security	3.00% Euro Notes due 2034
Trading Symbol	ITW34
Security Exchange Name	NYSE