| Form N-CSR Cover | 6 Months Ended |
| Dec. 21, 2024 | |
| Shareholder Report [Line Items] | |
| Document Type | N-CSRS |
| Amendment Flag | false |
| Registrant Name | BlackRock Financial Institutions Series Trust |
| Entity Central Index Key | 0000353281 |
| Entity Investment Company Type | N-1A |
| Document Period End Date | Oct. 31, 2024 |
| Shareholder Report | 6 Months Ended | |
| Dec. 21, 2024 USD ($) Holding | ||
| Shareholder Report [Line Items] | ||
| Document Type | N-CSRS | |
| Amendment Flag | false | |
| Registrant Name | BlackRock Financial Institutions Series Trust | |
| Entity Central Index Key | 0000353281 | |
| Entity Investment Company Type | N-1A | |
| Document Period End Date | Oct. 31, 2024 | |
| C000213594 [Member] | ||
| Shareholder Report [Line Items] | ||
| Fund Name | BlackRock Summit Cash Reserves Fund | |
| Class Name | Institutional Shares | |
| Trading Symbol | MSIXX | |
| Annual or Semi-Annual Statement [Text Block] | This semi-annual shareholder report contains important information about BlackRock Summit Cash Reserves Fund (the “Fund”) for the period of May 1, 2024 to October 31, 2024. | |
| Shareholder Report Annual or Semi-Annual | semi-annual shareholder report | |
| Additional Information [Text Block] | You can find additional information about the Fund at blackrock.com/fundreports | |
| Additional Information Phone Number | (800) 441‑7762 | |
| Additional Information Website | blackrock.com/fundreports | |
| Expenses [Text Block] | What were the Fund costs for the last six months ? (based on a hypothetical $10,000 investment) Class name Costs of a $10,000 Costs paid as a percentage of a Institutional Shares $ 0.42 % (a) (a) Annualized. | |
| Expenses Paid, Amount | $ 21 | |
| Expense Ratio, Percent | 0.42% | [1] |
| Performance Past Does Not Indicate Future [Text] | Past performance is not an indication of future results. | |
| Updated Performance Information Location [Text Block] | Visit blackrock.com . | |
| Net Assets | $ 951,517,981 | |
| Holdings Count | Holding | 158 | |
| Additional Fund Statistics [Text Block] | Key Fund statistics Net Assets $ Number of Portfolio Holdings 158 Current seven-day yields as of October 31, 2024 (The 7-day SEC Yield may differ from the 7-Day Yield shown due to 7-Day SEC Yield 4.50 % 7-Day Yield 4.50 % | |
| Holdings [Text Block] | What did the Fund invest in? (as of October 31, 2024) Portfolio composition Asset Type Percent of U.S. Treasury Obligations 49.9 % Repurchase Agreements 34.1 % U.S. Government Sponsored Agency Securities 19.1 % Liabilities in Excess of Other Assets (3.1 ) | |
| C000011303 [Member] | ||
| Shareholder Report [Line Items] | ||
| Fund Name | BlackRock Summit Cash Reserves Fund | |
| Class Name | Investor A Shares | |
| Trading Symbol | MSAXX | |
| Annual or Semi-Annual Statement [Text Block] | This semi-annual shareholder report contains important information about BlackRock Summit Cash Reserves Fund (the “Fund”) for the period of May 1, 2024 to October 31, 2024. | |
| Shareholder Report Annual or Semi-Annual | semi-annual shareholder report | |
| Additional Information [Text Block] | You can find additional information about the Fund at blackrock.com/fundreports | |
| Additional Information Phone Number | (800) 441‑7762 | |
| Additional Information Website | blackrock.com/fundreports | |
| Expenses [Text Block] | What were the Fund costs for the last six months ? (based on a hypothetical $10,000 investment) Class name Costs of a $10,000 Costs paid as a percentage of a Investor A Shares $ 0.42 % (a) (a) Annualized. | |
| Expenses Paid, Amount | $ 21 | |
| Expense Ratio, Percent | 0.42% | [2] |
| Performance Past Does Not Indicate Future [Text] | Past performance is not an indication of future results. | |
| Updated Performance Information Location [Text Block] | Visit blackrock.com . | |
| Net Assets | $ 951,517,981 | |
| Holdings Count | Holding | 158 | |
| Additional Fund Statistics [Text Block] | Key Fund statistics Net Assets $ Number of Portfolio Holdings 158 Current seven-day yields as of October 31, 2024 (The 7-day SEC Yield may differ from the 7-Day Yield shown due to 7-Day SEC Yield 4.50 % 7-Day Yield 4.50 % | |
| Holdings [Text Block] | What did the Fund invest in? (as of October 31, 2024) Portfolio composition Asset Type Percent of U.S. Treasury Obligations 49.9 % Repurchase Agreements 34.1 % U.S. Government Sponsored Agency Securities 19.1 % Liabilities in Excess of Other Assets (3.1 ) | |
| C000199555 [Member] | ||
| Shareholder Report [Line Items] | ||
| Fund Name | BlackRock Summit Cash Reserves Fund | |
| Class Name | Investor C Shares | |
| Trading Symbol | MCBXX | |
| Annual or Semi-Annual Statement [Text Block] | This semi-annual shareholder report contains important information about BlackRock Summit Cash Reserves Fund (the “Fund”) for the period of May 1, 2024 to October 31, 2024. | |
| Shareholder Report Annual or Semi-Annual | semi-annual shareholder report | |
| Additional Information [Text Block] | You can find additional information about the Fund at blackrock.com/fundreports | |
| Additional Information Phone Number | (800) 441‑7762 | |
| Additional Information Website | blackrock.com/fundreports | |
| Expenses [Text Block] | What were the Fund costs for the last six months ? (based on a hypothetical $10,000 investment) Class name Costs of a $10,000 Costs paid as a percentage of a Investor C Shares $ 1.17 % (a) (a) Annualized. | |
| Expenses Paid, Amount | $ 60 | |
| Expense Ratio, Percent | 1.17% | [3] |
| Performance Past Does Not Indicate Future [Text] | Past performance is not an indication of future results. | |
| Updated Performance Information Location [Text Block] | Visit blackrock.com . | |
| Net Assets | $ 951,517,981 | |
| Holdings Count | Holding | 158 | |
| Additional Fund Statistics [Text Block] | Key Fund statistics Net Assets $ Number of Portfolio Holdings 158 Current seven-day yields as of October 31, 2024 (The 7-day SEC Yield may differ from the 7-Day Yield shown due to 7-Day SEC Yield 3.75 % 7-Day Yield 3.75 % | |
| Holdings [Text Block] | What did the Fund invest in? (as of October 31, 2024) Portfolio composition Asset Type Percent of U.S. Treasury Obligations 49.9 % Repurchase Agreements 34.1 % U.S. Government Sponsored Agency Securities 19.1 % Liabilities in Excess of Other Assets (3.1 ) | |
| [1]Annualized.[2]Annualized.[3]Annualized. | ||
| Shareholder Report, Holdings (Details) | Dec. 21, 2024 |
| C000213594 [Member] | U S Treasury Obligations [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 49.90% |
| C000213594 [Member] | Repurchase Agreements [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 34.10% |
| C000213594 [Member] | U S Government Sponsored Agency Securities [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 19.10% |
| C000213594 [Member] | Liabilities in Excess of Other Assets [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | (3.10%) |
| C000011303 [Member] | U S Treasury Obligations [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 49.90% |
| C000011303 [Member] | Repurchase Agreements [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 34.10% |
| C000011303 [Member] | U S Government Sponsored Agency Securities [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 19.10% |
| C000011303 [Member] | Liabilities in Excess of Other Assets [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | (3.10%) |
| C000199555 [Member] | U S Treasury Obligations [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 49.90% |
| C000199555 [Member] | Repurchase Agreements [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 34.10% |
| C000199555 [Member] | U S Government Sponsored Agency Securities [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 19.10% |
| C000199555 [Member] | Liabilities in Excess of Other Assets [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | (3.10%) |
