Senior Notes - Additional Information (Detail)	Sep. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)	May 07, 2015 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	$ 131,000,000			$ 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	$ 185,000,000	$ 245,000,000		$ 245,000,000
Number of series of Senior Notes issued | Contract				4