Principal Amounts Carrying Amounts and Fair Values of Senior Notes (Parenthetical) (Detail) (USD $)	Sep. 30, 2014 May 2008 senior notes	Sep. 30, 2013 May 2008 senior notes	May 07, 2008 May 2008 senior notes	Sep. 30, 2014 July 2010 senior notes	Sep. 30, 2013 July 2010 senior notes	Jul. 14, 2010 July 2010 senior notes
Debt Instrument [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