Investments - Amortized Cost and Fair Value of Fixed Maturity Available-for-Sale Securities (Detail) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Jun. 30, 2013
Schedule of Available-for-sale Securities [Line Items]
Document Period End Date	Jun. 30, 2013
Total fixed maturity available-for-sale securities, Amortized Cost	$ 15,185.3
Total fixed maturity available-for-sale securities, Fair Value	15,578.3
Asset-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Other securities which provide for periodic payments, Amortized Cost	1,416.2
Other securities which provide for periodic payments, Fair Value	1,437.5
Commercial mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Other securities which provide for periodic payments, Amortized Cost	496.5
Other securities which provide for periodic payments, Fair Value	523.9
Structured hybrids [Member]
Schedule of Available-for-sale Securities [Line Items]
Other securities which provide for periodic payments, Amortized Cost	59.3
Other securities which provide for periodic payments, Fair Value	62.1
Agency residential mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Other securities which provide for periodic payments, Amortized Cost	108.7
Other securities which provide for periodic payments, Fair Value	111.0
Non-agency residential mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Other securities which provide for periodic payments, Amortized Cost	1,274.8
Other securities which provide for periodic payments, Fair Value	1,353.9
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	394.5
Due after one year through five years, Amortized Cost	2,720.5
Due after five years through ten years, Amortized Cost	3,468.3
Due after ten years, Amortized Cost	5,246.5
Subtotal, Amortized Cost	11,829.8
Due in one year or less, Fair Value	398.0
Due after one year through five years, Fair Value	2,783.6
Due after five years through ten years, Fair Value	3,528.8
Due after ten years, Fair Value	5,379.5
Subtotal, Fair Value	$ 12,089.9