| Document and Entity Information | Feb. 05, 2020 |
| Document And Entity Information [Line Items] | |
| Entity Registrant Name | ILLINOIS TOOL WORKS INC |
| Amendment Flag | false |
| Entity Central Index Key | 0000049826 |
| Document Type | 8-K |
| Document Period End Date | Feb. 5, 2020 |
| Entity Incorporation State Country Code | DE |
| Entity File Number | 1-4797 |
| 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 |
| Written Communications | false |
| Soliciting Material | false |
| Pre Commencement Tender Offer | false |
| Pre Commencement Issuer Tender Offer | false |
| Entity Emerging Growth Company | false |
| Common Stock [Member] | |
| Document And Entity Information [Line Items] | |
| Security 12b Title | Common Stock |
| Trading Symbol | ITW |
| Security Exchange Name | NYSE |
| A 1.75 Euro Notesdue 2022 [Member] | |
| Document And Entity Information [Line Items] | |
| Security 12b Title | 1.75% Euro Notes due 2022 |
| Trading Symbol | ITW22 |
| Security Exchange Name | NYSE |
| A 1.25 Euro Notesdue 2023 [Member] | |
| Document And Entity Information [Line Items] | |
| Security 12b Title | 1.25% Euro Notes due 2023 |
| Trading Symbol | ITW23 |
| Security Exchange Name | NYSE |
| A 0.250 Euro Notesdue 2024 [Member] | |
| Document And Entity Information [Line Items] | |
| Security 12b Title | 0.250% Euro Notes due 2024 |
| Trading Symbol | ITW24A |
| Security Exchange Name | NYSE |
| A 0.625 Euro Notesdue 2027 [Member] | |
| Document And Entity Information [Line Items] | |
| Security 12b Title | 0.625% Euro Notes due 2027 |
| Trading Symbol | ITW27 |
| Security Exchange Name | NYSE |
| A 2.125 Euro Notesdue 2030 [Member] | |
| Document And Entity Information [Line Items] | |
| Security 12b Title | 2.125% Euro Notes due 2030 |
| Trading Symbol | ITW30 |
| Security Exchange Name | NYSE |
| A 1.00 Euro Notesdue 2031 [Member] | |
| Document And Entity Information [Line Items] | |
| Security 12b Title | 1.00% Euro Notes due 2031 |
| Trading Symbol | ITW31 |
| Security Exchange Name | NYSE |
| A 3.00 Euro Notesdue 2034 [Member] | |
| Document And Entity Information [Line Items] | |
| Security 12b Title | 3.00% Euro Notes due 2034 |
| Trading Symbol | ITW34 |
| Security Exchange Name | NYSE |
