Investments - Net Investment Gains (Losses) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended	6 Months Ended
Mar. 31, 2015	Mar. 31, 2014	Mar. 31, 2015	Mar. 31, 2014
Gain (Loss) on Investments [Abstract]
Net realized gains on fixed maturity available-for-sale securities	$ 7.7	$ 7.9	$ 5.9	$ 17.7
Realized gains on equity securities	(6.4)hrg_GainLossOnSaleOfEquitySecurities	5.4hrg_GainLossOnSaleOfEquitySecurities	(3.2)hrg_GainLossOnSaleOfEquitySecurities	10.8hrg_GainLossOnSaleOfEquitySecurities
Net realized gains on securities	1.3us-gaap_GainLossOnSaleOfSecuritiesNet	13.3us-gaap_GainLossOnSaleOfSecuritiesNet	2.7us-gaap_GainLossOnSaleOfSecuritiesNet	28.5us-gaap_GainLossOnSaleOfSecuritiesNet
Realized gains (losses) on certain derivative instruments	43.2us-gaap_GainLossOnSaleOfDerivatives	50.0us-gaap_GainLossOnSaleOfDerivatives	88.2us-gaap_GainLossOnSaleOfDerivatives	110.6us-gaap_GainLossOnSaleOfDerivatives
Unrealized gains on certain derivative instruments	(51.2)us-gaap_UnrealizedGainLossOnDerivatives	(27.5)us-gaap_UnrealizedGainLossOnDerivatives	(50.7)us-gaap_UnrealizedGainLossOnDerivatives	39.3us-gaap_UnrealizedGainLossOnDerivatives
Embedded Derivative, Gain (Loss) on Embedded Derivative, Net	0.4	0	1.0	0
Change in fair value of derivatives	(7.6)hrg_ChangeInFairValueOfDerivatives	22.5hrg_ChangeInFairValueOfDerivatives	38.5hrg_ChangeInFairValueOfDerivatives	149.9hrg_ChangeInFairValueOfDerivatives
Realized gains on other invested assets	2.4	5.1	6.9	4.4
Net investment gains (losses)	$ (3.9)us-gaap_GainLossOnInvestments	$ 40.9us-gaap_GainLossOnInvestments	$ 48.1us-gaap_GainLossOnInvestments	$ 182.8us-gaap_GainLossOnInvestments

Change in fair value of derivatives.

No definition available.

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- Definition

Gain loss on sale of available for sale securities.

+ References

No definition available.

+ Details

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- Definition

Gain loss on sale of equity securities.

+ References

No definition available.

+ Details

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- Definition

Net Increase or Decrease in the fair value of the embedded derivative or group of embedded derivatives included in earnings in the period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 50

-Paragraph 4C

-URI http://asc.fasb.org/extlink&oid=56946850&loc=SL5624171-113959

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- Definition

This item represents the net total realized and unrealized gain (loss) included in earnings for the period as a result of selling or holding marketable securities categorized as trading, available-for-sale, or held-to-maturity, including the unrealized holding gain (loss) of held-to-maturity securities transferred to the trading security category and the cumulative unrealized gain (loss) which was included in other comprehensive income (a separate component of shareholders' equity) for available-for-sale securities transferred to trading securities during the period. Additionally, this item would include any gains (losses) realized during the period from the sale of investments accounted for under the cost method of accounting and losses recognized for other than temporary impairments (OTTI) of the subject investments.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 28

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 225

-SubTopic 10

-Section S99

-Paragraph 2

-Subparagraph (SX 210.5-03.7(c),9(a))

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 10

-Subparagraph (c)

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- Definition

The difference between the book value and the sale price of options, swaps, futures, forward contracts, and other derivative instruments. This element refers to the gain (loss) included in earnings.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 28

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=56944662&loc=d3e3602-108585

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 225

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-04.13(h))

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- Definition

The difference between the book value and the sale price of other nonspecific investments. This element is used when other, more specific, elements are not appropriate. This element refers to the gain (loss) included in earnings.

+ References

+ Details

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- Definition

The net gain (loss) realized from the sale, exchange, redemption, or retirement of securities, not separately or otherwise categorized as trading, available-for-sale, or held-to-maturity.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 225

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-04.13(h))

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 225

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-Paragraph 1

-Subparagraph (SX 210.7-04.3)

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- Definition

The net change in the difference between the fair value and the carrying value, or in the comparative fair values, of derivative instruments, including options, swaps, futures, and forward contracts, held at each balance sheet date, that was included in earnings for the period.

+ References

+ Details

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