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Senior Notes - Additional Information (Detail) (USD $)
In Millions, unless otherwise specified
0 Months Ended
Mar. 31, 2014
Sep. 30, 2013
May 07, 2008
May 2008 senior notes
Contract
May 07, 2008
May 2008 senior notes
Minimum
May 07, 2008
May 2008 senior notes
Maximum
Jul. 14, 2010
July 2010 senior notes
Contract
Jul. 14, 2010
July 2010 senior notes
Minimum
Jul. 14, 2010
July 2010 senior notes
Maximum
Debt Instrument [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   6 years 10 years
Senior Notes, weighted average interest rate     7.00%     5.20%    
Senior Notes, weighted average maturity (in years)     8 years 9 months 18 days     8 years    
Carrying value of Senior Notes 455.0 455.0            
Fair value of Senior Notes $ 466.6 $ 462.4