Vanguard Institutional Index Fund Average Annual Total Returns - Institutional Prospectus [Member]	12 Months Ended	60 Months Ended	120 Months Ended
	Dec. 31, 2025	Dec. 31, 2025	Dec. 31, 2025
Standard & Poor's 500 Index (reflects no deduction for fees, expenses, or taxes)[Member]
Prospectus [Line Items]
Average Annual Return, Percent	17.88%	14.42%	14.82%
Dow Jones U.S. Total Stock Market Float Adjusted Index (reflects no deduction for fees, expenses, or taxes)[Member]
Prospectus [Line Items]
Average Annual Return, Percent	17.05%	13.07%	14.21%
Institutional Shares
Prospectus [Line Items]
Average Annual Return, Percent	17.84%	14.39%	14.79%
Institutional Shares \| After Taxes on Distributions
Prospectus [Line Items]
Average Annual Return, Percent	17.21%	13.50%	13.98%
Institutional Shares \| After Taxes on Distributions and Sales
Prospectus [Line Items]
Average Annual Return, Percent	10.94%	11.34%	12.21%
Institutional Plus Shares
Prospectus [Line Items]
Average Annual Return, Percent	17.86%	14.40%	14.80%