Commitments and Contingencies - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016
Commitment And Contingencies [Line Items]
Reserves for legal, environmental and regulatory matters	$ 7.0
Long-term Debt	$ 5,563.4	$ 5,540.7
Maximum [Member]
Commitment And Contingencies [Line Items]
Expiry date of remainder part of notional amount	1 year
Fidelity And Guaranty Life Holdings [Member]
Commitment And Contingencies [Line Items]
Loss Contingency Accrual, Insurance-related Assessment, Premium Tax Offset		2.2
Loss Contingency, Undiscounted Amount of Insurance-related Assessment Liability		$ 2.1
FGL [Member]
Commitment And Contingencies [Line Items]
Reserves for legal, environmental and regulatory matters	$ 2.6
Expiration of partial amount of unfunded, legally binding lending commitments	143.8
Dale R. Ludwick, on behalf of Herself and All Others Similarly Situated v. Harbinger Group Inc., Fidelity & Guaranty Life Insurance Company, Raven Reinsurance Company, and Front Street Re (Cayman) Ltd [Member]
Commitment And Contingencies [Line Items]
Estimated Litigation Liability	1.7
Cressy Plaintiff [Member]
Commitment And Contingencies [Line Items]
Litigation Settlement, Amount	1.3
Loss Contingency Accrual, Provision	9.2
Estimated Litigation Liability	0.1
Eddie L. Cressy v. Fidelity Guaranty [sic] Life Insurance Company, et. al. [Member]
Commitment And Contingencies [Line Items]
Legal Fees	5.9
Litigation Settlement, Expense	3.3
Asset based loan receivables [Member] | Salus Capital Partners L L C [Member]
Commitment And Contingencies [Line Items]
Expiration of partial amount of unfunded, legally binding lending commitments	$ 2.3