Goodwill and Intangibles - Summary of Changes in Carrying Amounts of Goodwill and Intangible Assets Including FGL's DAC and VOBA Balances (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Goodwill And Intangible Assets [Line Items]
Document Period End Date			Jun. 30, 2017
Intangible Assets, Purchase Accounting Adjustments			$ 146.9
Intangible Assets, Including Goodwill [Roll Forward]
Beginning Balance, Goodwill			2,478.4
Beginning Balance, Indefinite Lived Intangible Assets			1,473.5
Beginning Balance, Finite Lived Intangible Assets			899.0
Beginning Balance, Intangible Assets, Excluding Goodwill			2,372.5
Less: Components of amortization:
Amortization of intangibles			(70.9)	$ (70.5)
Goodwill, Foreign Currency Translation Gain (Loss)			3.7
Effect of translation, Intangible Assets Excluding Goodwill			4.9
Ending Balance, Goodwill	$ 2,621.3		2,621.3
Ending Balance, Indefinite Lived Intangible Assets	1,558.1		1,558.1
Ending Balance, Finite Lived Intangible Assets	895.3		895.3
Ending Balance, Intangible Assets, Excluding Goodwill	2,453.4		2,453.4
Periodic amortization [Member]
Less: Components of amortization:
Amortization of intangibles			(70.9)
Finite-Lived Intangible Assets [Member]
Goodwill And Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Purchase Accounting Adjustments			65.8
Intangible Assets, Including Goodwill [Roll Forward]
Beginning Balance, Finite Lived Intangible Assets			899.0
Less: Components of amortization:
Amortization of intangibles	(23.8)	$ (23.5)	(70.9)	$ (70.5)
Effect of translation, Finite Lived intangible assets			1.4
Ending Balance, Finite Lived Intangible Assets	$ 895.3		895.3
Finite-Lived Intangible Assets [Member] | Periodic amortization [Member]
Less: Components of amortization:
Amortization of intangibles			(70.9)
Indefinite Lived Intangible Assets [Member]
Goodwill And Intangible Assets [Line Items]
Indefinite-lived Intangible Assets, Purchase Accounting Adjustments			81.1
Less: Components of amortization:
Indefinite-lived Intangible Assets, Translation Adjustments			3.5
Accounting Standards Update 2015-16 [Member] | Goodwill [Member]
Goodwill And Intangible Assets [Line Items]
Goodwill, Purchase Accounting Adjustments			$ 139.2