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, 2016	Mar. 31, 2015	Mar. 31, 2016	Mar. 31, 2015
Goodwill And Intangible Assets [Line Items]
Intangible Assets, Purchase Accounting Adjustments			$ 0.0
Intangible Assets, Including Goodwill [Roll Forward]
Beginning Balance, Goodwill			2,487.4
Beginning Balance, Indefinite Lived Intangible Assets			1,490.3
Beginning Balance, Finite Lived Intangible Assets			990.0
Beginning Balance, Intangible Assets, Excluding Goodwill			2,480.3
Impairment of Intangible Assets, Finite-lived			0.0	$ 60.2
Less: Components of amortization:
Amortization of intangibles	$ (23.4)	$ (21.2)	(47.0)	(41.7)
Effect of translation, Intangible Assets Excluding Goodwill			(0.9)
Ending Balance, Goodwill	2,494.1		2,494.1
Ending Balance, Indefinite Lived Intangible Assets	1,487.7		1,487.7
Ending Balance, Finite Lived Intangible Assets	944.7		944.7
Ending Balance, Intangible Assets, Excluding Goodwill	2,432.4		2,432.4
Periodic amortization [Member]
Less: Components of amortization:
Amortization of intangibles			(47.0)
Finite-Lived Intangible Assets [Member]
Goodwill And Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Purchase Accounting Adjustments			0.0
Intangible Assets, Including Goodwill [Roll Forward]
Beginning Balance, Finite Lived Intangible Assets			990.0
Less: Components of amortization:
Amortization of intangibles	(23.4)	$ (21.2)	(47.0)	$ (41.7)
Effect of translation, Finite Lived intangible assets			1.7
Ending Balance, Finite Lived Intangible Assets	$ 944.7		944.7
Finite-Lived Intangible Assets [Member] | Periodic amortization [Member]
Less: Components of amortization:
Amortization of intangibles			(47.0)
Goodwill [Member]
Goodwill And Intangible Assets [Line Items]
Goodwill, Purchase Accounting Adjustments			2.6
Less: Components of amortization:
Effect of translation, Finite Lived intangible assets			4.1
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			$ (2.6)