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		6 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 2017	Mar. 31, 2016
Goodwill And Intangible Assets [Line Items]
Document Period End Date			Mar. 31, 2017
Intangible Assets, Purchase Accounting Adjustments			$ 1.2
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			(47.1)	$ (47.0)
Goodwill, Foreign Currency Translation Gain (Loss)			(4.6)
Effect of translation, Intangible Assets Excluding Goodwill			(14.1)
Ending Balance, Goodwill	$ 2,473.8		2,473.8
Ending Balance, Indefinite Lived Intangible Assets	1,463.8		1,463.8
Ending Balance, Finite Lived Intangible Assets	848.7		848.7
Ending Balance, Intangible Assets, Excluding Goodwill	2,312.5		2,312.5
Periodic amortization [Member]
Less: Components of amortization:
Amortization of intangibles			(47.1)
Finite-Lived Intangible Assets [Member]
Goodwill And Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Purchase Accounting Adjustments			1.2
Intangible Assets, Including Goodwill [Roll Forward]
Beginning Balance, Finite Lived Intangible Assets			899.0
Less: Components of amortization:
Amortization of intangibles	(23.5)	$ (23.4)	(47.1)	$ (47.0)
Effect of translation, Finite Lived intangible assets			(4.4)
Ending Balance, Finite Lived Intangible Assets	$ 848.7		848.7
Finite-Lived Intangible Assets [Member] | Periodic amortization [Member]
Less: Components of amortization:
Amortization of intangibles			(47.1)
Indefinite Lived Intangible Assets [Member]
Goodwill And Intangible Assets [Line Items]
Indefinite-lived Intangible Assets, Purchase Accounting Adjustments			0.0
Less: Components of amortization:
Indefinite-lived Intangible Assets, Translation Adjustments			(9.7)
Accounting Standards Update 2015-16 [Member] | Goodwill [Member]
Goodwill And Intangible Assets [Line Items]
Goodwill, Purchase Accounting Adjustments			$ 0.0