v3.25.2
Form N-1A Cover
Apr. 30, 2025
Prospectus [Line Items]  
Document Type 485BPOS
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 Apr. 30, 2025
Prospectus Date Aug. 28, 2025