Transfers of Assets between Level 1 and Level 2 (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Fixed Maturity Securities
Fair Value, Assets, Level 2 to Level 1 Transfers, Amount	$ 1,115.7	$ 2,006.1
Fair Value, Assets, Level 1 to Level 2 Transfers, Amount	3,730.5	3,262.5
United States Government and Government Agencies and Authorities
Fixed Maturity Securities
Fair Value, Assets, Level 2 to Level 1 Transfers, Amount	0	169.8
Fair Value, Assets, Level 1 to Level 2 Transfers, Amount	224.7	0
States, Municipalities, and Political Subdivisions
Fixed Maturity Securities
Fair Value, Assets, Level 2 to Level 1 Transfers, Amount	0	47.7
Fair Value, Assets, Level 1 to Level 2 Transfers, Amount	43.8	274.9
Foreign Government Debt Securities [Member]
Fixed Maturity Securities
Fair Value, Assets, Level 2 to Level 1 Transfers, Amount	0	0
Fair Value, Assets, Level 1 to Level 2 Transfers, Amount	0	0.7
Public Utility, Bonds [Member]
Fixed Maturity Securities
Fair Value, Assets, Level 2 to Level 1 Transfers, Amount	47.1	362.1
Fair Value, Assets, Level 1 to Level 2 Transfers, Amount	653.1	534.2
All Other Corporate Bonds
Fixed Maturity Securities
Fair Value, Assets, Level 2 to Level 1 Transfers, Amount	1,068.6	1,426.5
Fair Value, Assets, Level 1 to Level 2 Transfers, Amount	$ 2,808.9	$ 2,452.7