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Investments - Fair Value and Gross Unrealized Losses of Available-for-Sale Securities (Detail)
$ in Millions
9 Months Ended
Jun. 30, 2015
USD ($)
Security
Sep. 30, 2014
USD ($)
Security
Schedule of Available-for-sale Securities [Line Items]    
Document Period End Date Jun. 30, 2015  
Available-for-sale securities, Fair Value, Less than 12 months $ 5,297.0 $ 2,577.1
Available For Sale Securities In A Continuous Unrealized Loss Position Less Than 12 Months Aggregate Losses (176.4) (39.4)
Available-for-sale securities, Fair Value, 12 months or longer 2,519.9 2,285.5
Available-for-sale Securities, Gross Unrealized Losses, 12 months or longer (106.5) (69.4)
Available-for-sale securities, Fair Value, Total 7,816.9 4,862.6
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss $ 282.9 $ 108.8
Total number of available-for-sale securities in an unrealized loss position, Less than 12 months | Security 783 319
Total number of available-for-sale securities in an unrealized loss position, 12 months or longer | Security 300 310
Total number of available-for-sale securities in an unrealized loss position | Security 1,083 629
Asset-backed Securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale securities, Fair Value, Less than 12 months $ 725.9 $ 825.8
Available For Sale Securities In A Continuous Unrealized Loss Position Less Than 12 Months Aggregate Losses (10.2) (11.8)
Available-for-sale securities, Fair Value, 12 months or longer 597.4 288.2
Available-for-sale Securities, Gross Unrealized Losses, 12 months or longer (13.9) (7.0)
Available-for-sale securities, Fair Value, Total 1,323.3 1,114.0
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss 24.1 18.8
Commercial Mortgage Backed Securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale securities, Fair Value, Less than 12 months 318.9 160.3
Available For Sale Securities In A Continuous Unrealized Loss Position Less Than 12 Months Aggregate Losses (6.9) (0.9)
Available-for-sale securities, Fair Value, 12 months or longer 56.8 0.4
Available-for-sale Securities, Gross Unrealized Losses, 12 months or longer (0.1) (1.1)
Available-for-sale securities, Fair Value, Total 375.7 160.7
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss 7.0 2.0
Corporate Debt Securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale securities, Fair Value, Less than 12 months 2,832.7 816.6
Available For Sale Securities In A Continuous Unrealized Loss Position Less Than 12 Months Aggregate Losses (127.3) (16.3)
Available-for-sale securities, Fair Value, 12 months or longer 893.8 1,127.8
Available-for-sale Securities, Gross Unrealized Losses, 12 months or longer (46.3) (32.6)
Available-for-sale securities, Fair Value, Total 3,726.5 1,944.4
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss 173.6 48.9
Equities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale securities, Fair Value, Less than 12 months 59.9 180.4
Available For Sale Securities In A Continuous Unrealized Loss Position Less Than 12 Months Aggregate Losses (1.3) (2.2)
Available-for-sale securities, Fair Value, 12 months or longer 94.6 54.9
Available-for-sale Securities, Gross Unrealized Losses, 12 months or longer (3.1) (2.9)
Available-for-sale securities, Fair Value, Total 154.5 235.3
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss 4.4 5.1
Hybrids [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale securities, Fair Value, Less than 12 months 174.1 258.2
Available For Sale Securities In A Continuous Unrealized Loss Position Less Than 12 Months Aggregate Losses (5.2) (2.3)
Available-for-sale securities, Fair Value, 12 months or longer 360.9 290.0
Available-for-sale Securities, Gross Unrealized Losses, 12 months or longer (23.7) (12.9)
Available-for-sale securities, Fair Value, Total 535.0 548.2
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss 28.9 15.2
Municipals [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale securities, Fair Value, Less than 12 months 339.4  
Available For Sale Securities In A Continuous Unrealized Loss Position Less Than 12 Months Aggregate Losses (12.8)  
Available-for-sale securities, Fair Value, 12 months or longer 210.9 264.9
Available-for-sale Securities, Gross Unrealized Losses, 12 months or longer (7.0) (6.3)
Available-for-sale securities, Fair Value, Total 550.3 264.9
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss 19.8 6.3
Agency residential mortgage-backed securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale securities, Fair Value, Less than 12 months 496.2 298.5
Available For Sale Securities In A Continuous Unrealized Loss Position Less Than 12 Months Aggregate Losses (12.7) (5.8)
Available-for-sale securities, Fair Value, 12 months or longer 246.2 177.6
Available-for-sale Securities, Gross Unrealized Losses, 12 months or longer (12.0) (5.3)
Available-for-sale securities, Fair Value, Total 742.4 476.1
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss 24.7 11.1
U.S. Government securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale securities, Fair Value, Less than 12 months 349.9 37.3
Available For Sale Securities In A Continuous Unrealized Loss Position Less Than 12 Months Aggregate Losses   (0.1)
Available-for-sale securities, Fair Value, 12 months or longer 59.3 81.7
Available-for-sale Securities, Gross Unrealized Losses, 12 months or longer (0.4) (1.3)
Available-for-sale securities, Fair Value, Total 409.2 119.0
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss $ 0.4 $ 1.4