Senior Notes - Additional Information (Detail) (USD $)	Sep. 30, 2014 May 2008 senior notes	Sep. 30, 2013 May 2008 senior notes	May 07, 2008 May 2008 senior notes Contract	Sep. 30, 2014 July 2010 senior notes	Sep. 30, 2013 July 2010 senior notes	Jul. 14, 2010 July 2010 senior notes Contract
Debt Disclosure [Line Items]
Senior Notes issued in a private placement to a group of institutional investors	$ 202,000,000	$ 210,000,000	$ 275,000,000	$ 245,000,000	$ 245,000,000	$ 245,000,000
Number of series of Senior Notes issued			4			4
Senior Notes issued in a private placement to a group of institutional investors, annual principal payment	$ 8,000,000