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Senior Notes - Additional Information (Detail) (USD $)
In Millions, unless otherwise specified
Mar. 31, 2012
Sep. 30, 2011
May 31, 2008
May 2008 senior notes
Year
Contract
May 31, 2008
May 2008 senior notes
Minimum
May 31, 2008
May 2008 senior notes
Maximum
Jul. 31, 2010
July 2010 senior notes
Contract
Year
Jul. 31, 2010
July 2010 senior notes
Minimum
Jul. 31, 2010
July 2010 senior notes
Maximum
Debt Disclosure [Line Items]                
Senior Notes issued in a private placement to a group of institutional investors     $ 275     $ 245    
Number of series of Senior Notes issued     4     4    
Senior Notes, maturity       5 years 10 years   5 years 10 years
Senior Notes, weighted average interest rate     6.80%     5.20%    
Senior Notes, weighted average maturity (in years)     7.9     8.0    
Carrying value of Senior Notes 512.0 512.0            
Fair value of Senior Notes $ 517.3 $ 572.5