Investments - Amortized Cost and Fair Value of Fixed Maturity Available-for-Sale Securities (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Total fixed maturity available-for-sale securities, Amortized Cost	$ 15,200.0
Total fixed maturity available-for-sale securities, Fair Value	16,183.5
Asset-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Other securities which provide for periodic payments, Amortized Cost	1,388.5
Other securities which provide for periodic payments, Fair Value	1,418.4
Commercial mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Other securities which provide for periodic payments, Amortized Cost	506.8
Other securities which provide for periodic payments, Fair Value	546.2
Structured hybrids [Member]
Schedule of Available-for-sale Securities [Line Items]
Other securities which provide for periodic payments, Amortized Cost	83.6
Other securities which provide for periodic payments, Fair Value	87.1
Agency residential mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Other securities which provide for periodic payments, Amortized Cost	122.2
Other securities which provide for periodic payments, Fair Value	126.0
Non-agency residential mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Other securities which provide for periodic payments, Amortized Cost	1,209.5
Other securities which provide for periodic payments, Fair Value	1,298.5
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	615.2
Due after one year through five years, Amortized Cost	2,888.8
Due after five years through ten years, Amortized Cost	3,702.2
Due after ten years, Amortized Cost	4,683.2
Subtotal, Amortized Cost	11,889.4
Due in one year or less, Fair Value	619.4
Due after one year through five years, Fair Value	2,985.3
Due after five years through ten years, Fair Value	3,958.0
Due after ten years, Fair Value	5,144.6
Subtotal, Fair Value	$ 12,707.3