v3.26.1
Vanguard Ultra-Short Treasury ETF N-1A Cover
Dec. 31, 2025
Prospectus [Line Items]  
Document Type 485BPOS
Amendment Flag false
Registrant Name VANGUARD INSTITUTIONAL INDEX FUNDS
Entity Central Index Key 0000862084
Entity Investment Company Type N-1A
Document Period End Date Dec. 31, 2025
Prospectus Date Apr. 28, 2026