| Document and Entity Information | May 26, 2021 |
| Document And Entity Information [Line Items] | |
| Entity Registrant Name | BlackRock Inc. |
| Entity Incorporation State Country Code | DE |
| Entity Address, State or Province | NY |
| Amendment Flag | false |
| Entity Central Index Key | 0001364742 |
| Current Fiscal Year End Date | --12-31 |
| Document Type | 8-K |
| Document Period End Date | May 26, 2021 |
| Entity File Number | 001-33099 |
| Entity Tax Identification Number | 32-0174431 |
| Entity Address, Address Line One | 55 East 52nd Street |
| Entity Address, City or Town | New York |
| Entity Address, Postal Zip Code | 10055 |
| City Area Code | (212) |
| Local Phone Number | 810-5300 |
| 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, $.01 par value |
| Trading Symbol | BLK |
| Security Exchange Name | NYSE |
| Deferrable Notes [Member] | |
| Document And Entity Information [Line Items] | |
| Security 12b Title | 1.250% Notes due 2025 |
| Trading Symbol | BLK25 |
| Security Exchange Name | NYSE |
