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Senior Notes - Additional Information (Detail) (USD $)
In Millions, unless otherwise specified
0 Months Ended 0 Months Ended
Dec. 31, 2013
Sep. 30, 2013
May 07, 2008
May 2008 senior notes
Y
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
Y
Jul. 14, 2010
July 2010 senior notes
Minimum
Jul. 14, 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   6 years 10 years
Senior Notes, weighted average interest rate     7.00%     5.20%    
Senior Notes, weighted average maturity (in years)     8.8     8.0    
Carrying value of Senior Notes 455.0 455.0            
Fair value of Senior Notes $ 462.9 $ 462.4