Transfers of Assets between Level 1 and Level 2 (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Fixed Maturity Securities
Level 1 from Level 2	$ 3,869.0	$ 3,991.3
Level 2 from Level 1	2,386.9	2,297.5
United States Government and Government Agencies and Authorities
Fixed Maturity Securities
Level 1 from Level 2	0	16.1
Level 2 from Level 1	204.2	0
States, Municipalities, and Political Subdivisions
Fixed Maturity Securities
Level 1 from Level 2	0	25.4
Level 2 from Level 1	43.8	274.9
Foreign Governments
Fixed Maturity Securities
Level 1 from Level 2	0	0
Level 2 from Level 1	0	0.7
Public Utilities
Fixed Maturity Securities
Level 1 from Level 2	826.2	885.5
Level 2 from Level 1	454.4	481.1
All Other Corporate Bonds
Fixed Maturity Securities
Level 1 from Level 2	3,042.8	3,064.3
Level 2 from Level 1	$ 1,684.5	$ 1,540.8