Investments - Amortized Cost and Fair Value of Fixed Maturity Available-for-Sale Securities (Detail) (USD $) In Millions, unless otherwise specified	6 Months Ended
	Mar. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Document Period End Date	Mar. 31, 2014
Total fixed maturity available-for-sale securities, Amortized Cost	$ 16,040.5
Total fixed maturity available-for-sale securities, Fair Value	16,626.6
Commercial mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Other securities which provide for periodic payments, Amortized Cost	463.3
Other securities which provide for periodic payments, Fair Value	486.6
Structured hybrids [Member]
Schedule of Available-for-sale Securities [Line Items]
Other securities which provide for periodic payments, Amortized Cost	73.5
Other securities which provide for periodic payments, Fair Value	77.1
Agency residential mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Other securities which provide for periodic payments, Amortized Cost	96.9
Other securities which provide for periodic payments, Fair Value	99.4
Non-agency residential mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Other securities which provide for periodic payments, Amortized Cost	1,730.9
Other securities which provide for periodic payments, Fair Value	1,847.1
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	353.8
Due after one year through five years, Amortized Cost	2,770.7
Due after five years through ten years, Amortized Cost	3,528.3
Due after ten years, Amortized Cost	5,446.5
Subtotal, Amortized Cost	12,099.3
Due in one year or less, Fair Value	356.8
Due after one year through five years, Fair Value	2,856.0
Due after five years through ten years, Fair Value	3,612.6
Due after ten years, Fair Value	5,703.8
Subtotal, Fair Value	$ 12,529.2