Shareholder Fees (Vanguard Extended Duration Treasury Index Fund, USD $)	12 Months Ended
	Aug. 31, 2011
Vanguard Extended Duration Treasury Index Fund - ETF Shares
Shareholder Fees (fees paid directly from your investment]
Transaction Fee on Conversion to ETF Shares	0
Transaction Fee On Purchases and Sales	0
Transaction Fee On Reinvested Dividends	0
Vanguard Extended Duration Treasury Index Fund - Institutional Shares
Shareholder Fees (fees paid directly from your investment]
Sales charge (load) imposed on purchases	none
Purchase Fee	0.50%
Sales charge (load) imposed on reinvested dividends	none
Redemption Fee	none
Account Service Fee (for fund account balances below $10,000)	none
Vanguard Extended Duration Treasury Index Fund - Institutional Plus Shares
Shareholder Fees (fees paid directly from your investment]
Sales charge (load) imposed on purchases	none
Purchase Fee	0.50%
Sales charge (load) imposed on reinvested dividends	none
Redemption Fee	none
Account Service Fee (for fund account balances below $10,000)	none
Institutional
Shareholder Fees (fees paid directly from your investment]
Purchase Fee Parenthetical	(other than on reinvested dividends or capital gains)