Transfers of Assets between Level 1 and Level 2 (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010
Fixed Maturity Securities
Level 1 from Level 2	$ 2,006.1	$ 2,556.1
Level 2 from Level 1	3,262.5	2,389.4
United States Government and Government Agencies and Authorities
Fixed Maturity Securities
Level 1 from Level 2	169.8	95.9
Level 2 from Level 1	0	0
States, Municipalities, and Political Subdivisions
Fixed Maturity Securities
Level 1 from Level 2	47.7	36.2
Level 2 from Level 1	274.9	33.7
Foreign Governments
Fixed Maturity Securities
Level 1 from Level 2	0	0.7
Level 2 from Level 1	0.7	0
Public Utilities
Fixed Maturity Securities
Level 1 from Level 2	362.1	483.2
Level 2 from Level 1	534.2	673.6
All Other Corporate Bonds
Fixed Maturity Securities
Level 1 from Level 2	1,426.5	1,940.1
Level 2 from Level 1	2,452.7	1,676.6
Redeemable Preferred Stocks [Member]
Fixed Maturity Securities
Level 1 from Level 2	0	0
Level 2 from Level 1	$ 0	$ 5.5