Gross Unrealized Losses and Fair Value of Investments (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Sep. 30, 2012
Investments, Unrealized Loss Position [Line Items]
Less than 12 months, Fair Value	$ 15,000
Less than 12 months, Unrealized Losses	(1)
12 months or greater, Fair Value
12 months or greater, Unrealized Losses
Total, Fair Value	15,000
Total, Unrealized Losses	(1)
U.S. government obligations
Investments, Unrealized Loss Position [Line Items]
Less than 12 months, Fair Value	15,000
Less than 12 months, Unrealized Losses	(1)
12 months or greater, Fair Value
12 months or greater, Unrealized Losses
Total, Fair Value	15,000
Total, Unrealized Losses	$ (1)