Summary of Significant Accounting Policies and Select Balance Sheet Information - Sales of Available-for-Sale Securities (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2012
Investments, Debt and Equity Securities [Abstract]
Proceeds from sales	$ 162,673	$ 44,853	$ 43,556
Gross realized gains	134	179	229
Gross realized losses	$ (1)	$ (43)	$ (1)