Goodwill and Intangibles - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Sep. 30, 2016
Finite-Lived Intangible Assets [Line Items]
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent	35.00%
Amortization of intangibles	$ 23.6	$ 23.6
Finite-Lived Intangible Assets, Accumulated Amortization	508.5		$ 488.7
Finite-Lived Intangible Assets [Member]
Finite-Lived Intangible Assets [Line Items]
Amortization of intangibles	23.6
VOBA [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Amortization Expense, Next Twelve Months	91.9
2014	85.7
2015	85.4
2016	85.2
2017	81.9
Trade Names [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Accumulated Amortization	$ 93.0		$ 89.1
Minimum [Member] | Technology Assets [Member]
Finite-Lived Intangible Assets [Line Items]
Amortizable Life	5 years
Minimum [Member] | Trade Names [Member]
Finite-Lived Intangible Assets [Line Items]
Amortizable Life	5 years
Maximum [Member] | Technology Assets [Member]
Finite-Lived Intangible Assets [Line Items]
Amortizable Life	18 years
Maximum [Member] | Trade Names [Member]
Finite-Lived Intangible Assets [Line Items]
Amortizable Life	13 years
Weighted Average [Member] | Technology Assets [Member]
Finite-Lived Intangible Assets [Line Items]
Amortizable Life	11 years 2 months 6 days
Weighted Average [Member] | Trade Names [Member]
Finite-Lived Intangible Assets [Line Items]
Amortizable Life	11 years 4 months 12 days