Summary of Significant Accounting Policies and Select Balance Sheet Information - Sales of Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2013
Investments Debt And Equity Securities [Abstract]
Proceeds from sales	$ 22,199	$ 162,673	$ 44,853
Gross realized gains	548	134	179
Gross realized losses	$ (73)	$ (1)	$ (43)