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
	Dec. 31, 2016	Dec. 31, 2015
Goodwill And Intangible Assets [Line Items]
Document Period End Date	Dec. 31, 2016
Intangible Assets, Purchase Accounting Adjustments	$ 0.8
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	(23.6)	$ (23.6)
Goodwill, Foreign Currency Translation Gain (Loss)	(13.9)
Effect of translation, Intangible Assets Excluding Goodwill	(21.8)
Ending Balance, Goodwill	2,464.5
Ending Balance, Indefinite Lived Intangible Assets	1,459.6
Ending Balance, Finite Lived Intangible Assets	868.3
Ending Balance, Intangible Assets, Excluding Goodwill	2,327.9
Periodic amortization [Member]
Less: Components of amortization:
Amortization of intangibles	(23.6)
Finite-Lived Intangible Assets [Member]
Goodwill And Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Purchase Accounting Adjustments	0.8
Intangible Assets, Including Goodwill [Roll Forward]
Beginning Balance, Finite Lived Intangible Assets	899.0
Less: Components of amortization:
Amortization of intangibles	(23.6)
Effect of translation, Finite Lived intangible assets	(7.9)
Ending Balance, Finite Lived Intangible Assets	868.3
Finite-Lived Intangible Assets [Member] | Periodic amortization [Member]
Less: Components of amortization:
Amortization of intangibles	(23.6)
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	(13.9)
Accounting Standards Update 2015-16 [Member] | Goodwill [Member]
Goodwill And Intangible Assets [Line Items]
Goodwill, Purchase Accounting Adjustments	$ 0.0