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Senior Notes - Additional Information (Detail) (USD $)
In Millions
Jul. 14, 2010
May 07, 2008
Sep. 30, 2011
Senior Notes issued on May 7, 2008
Sep. 30, 2010
Senior Notes issued on May 7, 2008
Sep. 30, 2011
Senior Notes issued on July 14, 2010
Sep. 30, 2010
Senior Notes issued on July 14, 2010
Debt Disclosure [Line Items]      
Senior Notes issued in a private placement to a group of institutional investors$ 245$ 275    
Senior Notes issued in a private placement to a group of institutional investors, annual principal payment  8.0   
Fair value of Senior Notes  $ 305.9$ 315.0$ 266.6$ 258.7