Investments - Net Investment Gains (Losses) (Detail) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
Jun. 30, 2015	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2013
Gain (Loss) on Investments [Line Items]
Net realized gains on fixed maturity available-for-sale securities		$ 48.6	$ 101.3	$ 330.0
Realized (losses) gains on equity securities		(4.0)	13.5	12.6
Net realized gains on securities		44.6	114.8	342.6
Realized gains on certain derivative instruments		118.6	233.8	148.6
Unrealized (losses) gains on certain derivative instruments		(231.9)	37.7	20.5
Embedded Derivative, Gain (Loss) on Embedded Derivative, Net	$ (1.0)	(1.0)	(0.1)	0.0
Change in fair value of derivatives		(114.3)	271.4	169.1
Realized (losses) gains on other invested assets and funds withheld receivables		(35.0)	9.1	(0.1)
Net investment (losses) gains		(104.7)	395.3	511.6
Net Investment Income
Gain (Loss) on Investments [Line Items]
Available For Sale Securities On Fixed Maturity Under Commitment Agreement		4,776.6	5,033.4	8,986.9
Available-for-sale Securities, Gross Realized Gains		83.3	108.5	351.2
Available-for-sale Securities, Gross Realized Losses		$ 72.5	$ 4.9	$ 18.3

Available for sale securities on fixed maturity under commitment agreement.

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Change in fair value of derivatives.

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Gain loss on sale of available for sale securities.

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Gain loss on sale of equity securities.

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This item represents the gross profit realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities.

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This item represents the gross loss realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities.

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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

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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.

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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.

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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.

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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.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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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.

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