Senior Notes - Additional Information (Detail) (USD $)	12 Months Ended									1 Months Ended	12 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2013 May 2008 senior notes	Sep. 30, 2012 May 2008 senior notes	May 07, 2008 May 2008 senior notes	Sep. 30, 2013 July 2010 senior notes	Sep. 30, 2012 July 2010 senior notes	Jul. 14, 2010 July 2010 senior notes	May 31, 2008 Series A note	Sep. 30, 2013 Series A note
Debt Disclosure [Line Items]
Senior Notes issued in a private placement to a group of institutional investors				$ 210,000,000	$ 259,000,000	$ 275,000,000	$ 245,000,000	$ 245,000,000	$ 245,000,000
Senior Notes issued in a private placement to a group of institutional investors, annual principal payment				8,000,000
Senior notes maturity date										May 07, 2013	May 07, 2013
Payments on senior notes	$ 49,000,000	$ 8,000,000	$ 8,000,000								$ 41,000,000