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Fair Value of Financial Instruments - Assets and Liabilities Carrying at Fair Value on Recurring Basis (Detail) - USD ($)
$ in Millions
Mar. 31, 2018
Sep. 30, 2017
Mar. 31, 2017
Sep. 30, 2016
Assets        
Cash and cash equivalents $ 758.8 $ 270.1 $ 294.6 $ 465.2
Estimate of Fair Value Measurement [Member]        
Liabilities        
Debt Instrument, Fair Value Disclosure 5,441.5 5,931.0    
Reported Value Measurement [Member]        
Liabilities        
Debt Instrument, Fair Value Disclosure 5,318.7 5,705.1    
Fair Value, Inputs, Level 2 [Member]        
Liabilities        
Debt Instrument, Fair Value Disclosure 5,441.5 5,839.0    
Fair Value, Inputs, Level 3 [Member]        
Liabilities        
Debt Instrument, Fair Value Disclosure   92.0    
Fair Value, Measurements, Recurring | Estimate of Fair Value Measurement [Member]        
Fixed maturity securities, available-for-sale:        
Assets, Fair Value Disclosure, Recurring 3.7 1.5    
Derivatives, Liabilities        
Liabilities, Fair Value Disclosure, Recurring 1.1 3.3    
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1        
Fixed maturity securities, available-for-sale:        
Assets, Fair Value Disclosure, Recurring 0.0 0.0    
Derivatives, Liabilities        
Liabilities, Fair Value Disclosure, Recurring 0.0 0.0    
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 [Member]        
Fixed maturity securities, available-for-sale:        
Assets, Fair Value Disclosure, Recurring 3.7 1.5    
Derivatives, Liabilities        
Liabilities, Fair Value Disclosure, Recurring 1.1 3.3    
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 [Member]        
Fixed maturity securities, available-for-sale:        
Assets, Fair Value Disclosure, Recurring 0.0 0.0    
Derivatives, Liabilities        
Liabilities, Fair Value Disclosure, Recurring $ 0.0 $ 0.0