Goodwill and Intangibles, net - Summary of Amortization Expense of Definite Lived Intangible Assets (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Amortization Expense, Year Two	$ 57.5		$ 57.5
VOBA
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Amortization Expense, Year Two	57.4		57.4
Finite-Lived Intangible Assets, Amortization Expense, Year Three	55.0		55.0
Finite-Lived Intangible Assets, Amortization Expense, Year Four	49.7		49.7
Finite-Lived Intangible Assets, Amortization Expense, Year Five	48.0		48.0
Definite Lived Intangible Assets
Finite-Lived Intangible Assets [Line Items]
Amortization of intangibles	$ 14.4	$ 15.1	$ 29.4	$ 30.5