Investments - Amortized Cost and Fair Value of Fixed Maturity Available-for-Sale Securities (Detail) - USD ($) $ in Millions	9 Months Ended
Jun. 30, 2015	Sep. 30, 2014
Schedule of Available-for-sale Securities [Line Items]
Document Period End Date	Jun. 30, 2015
Total fixed maturity available-for-sale securities, Amortized Cost	$ 17,388.5
Total fixed maturity available-for-sale securities, Fair Value	17,723.6
Commercial Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Other securities which provide for periodic payments, Amortized Cost	814.2
Other securities which provide for periodic payments, Fair Value	821.6
Structured hybrids [Member]
Schedule of Available-for-sale Securities [Line Items]
Other securities which provide for periodic payments, Amortized Cost	673.3
Other securities which provide for periodic payments, Fair Value	661.1
Agency residential mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Other securities which provide for periodic payments, Amortized Cost	2,099.5
Other securities which provide for periodic payments, Fair Value	2,175.4
Other Invested Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Trading Securities, Cost		$ 164.9
Other Investments Gross Unrealized Gain		0.1
Trading Securities		$ 165.0
Non-structured Hybrids [Member] \| Corporate [Member] \| Municipal [Member] \| U.S. Government securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Due in one year or less, Amortized Cost	203.2
Due after one year through five years, Amortized Cost	2,113.2
Due after five years through ten years, Amortized Cost	2,955.1
Due after ten years, Amortized Cost	6,623.3
Subtotal, Amortized Cost	11,894.8
Due in one year or less, Fair Value	205.7
Due after one year through five years, Fair Value	2,153.2
Due after five years through ten years, Fair Value	3,015.0
Due after ten years, Fair Value	6,804.1
Subtotal, Fair Value	12,178.0
Other Investments Asset [Member] \| Other Invested Assets [Member] \| All Other Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Trading Securities, Cost	442.4
Other Investments Gross Unrealized Gain	0.0
Other Short Term Investments Gross Unrealized Losses	0.0
Trading Securities	$ 442.4

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