Transfers of Assets between Level 1 and Level 2 (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Fixed Maturity Securities
Level 1 from Level 2	$ 3,025.3	$ 3,869.0
Level 2 from Level 1	1,213.7	2,386.9
United States Government and Government Agencies and Authorities
Fixed Maturity Securities
Level 1 from Level 2	256.9	0
Level 2 from Level 1	0	204.2
States, Municipalities, and Political Subdivisions
Fixed Maturity Securities
Level 1 from Level 2	0	0
Level 2 from Level 1	19.4	43.8
Public Utilities
Fixed Maturity Securities
Level 1 from Level 2	502.1	826.2
Level 2 from Level 1	53.5	454.4
All Other Corporate Bonds
Fixed Maturity Securities
Level 1 from Level 2	2,266.3	3,042.8
Level 2 from Level 1	$ 1,140.8	$ 1,684.5