Investments - Sales of Available-for-Sale Securities (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Mar. 31, 2014	Mar. 31, 2013
Investments Debt And Equity Securities [Abstract]
Proceeds from sales	$ 89,647	$ 25,175	$ 92,514	$ 25,980
Gross realized gains	125	161	126	165
Gross realized losses			$ (1)