Senior Notes - Additional Information (Detail)	Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)	Jul. 14, 2010 USD ($) Contract	May. 07, 2008 USD ($) Contract
May 2008 senior notes
Debt Disclosure [Line Items]
Senior Notes issued in a private placement to a group of institutional investors	$ 131,000,000	$ 202,000,000		$ 275,000,000
Number of series of Senior Notes issued | Contract				4
Senior Notes issued in a private placement to a group of institutional investors, annual principal payment	8,000,000
July 2010 senior notes
Debt Disclosure [Line Items]
Senior Notes issued in a private placement to a group of institutional investors	$ 245,000,000	$ 245,000,000	$ 245,000,000
Number of series of Senior Notes issued | Contract			4