Transfers of Assets between Level 1 and Level 2 (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Fixed Maturity Securities
Level 1 from Level 2	$ 3,026.8	$ 2,641.0	$ 3,570.8	$ 3,239.8
Level 2 from Level 1	2,534.4	3,078.1	2,284.9	2,523.5
United States Government and Government Agencies and Authorities
Fixed Maturity Securities
Level 1 from Level 2	426.8	177.2	205.9	169.8
Level 2 from Level 1	60.5	16.1	20.5	0
States, Municipalities, and Political Subdivisions
Fixed Maturity Securities
Level 1 from Level 2	43.0	0	41.9	25.4
Level 2 from Level 1	62.9	20.0	74.8	301.9
Foreign Government Debt Securities
Fixed Maturity Securities
Level 1 from Level 2	21.7	0	20.7	0
Level 2 from Level 1	0	0	0	0.7
Public Utilities
Fixed Maturity Securities
Level 1 from Level 2	382.4	665.7	479.0	710.5
Level 2 from Level 1	743.6	694.8	483.6	488.6
All Other Corporate Bonds
Fixed Maturity Securities
Level 1 from Level 2	2,152.9	1,798.1	2,823.3	2,334.1
Level 2 from Level 1	1,667.4	2,347.2	1,706.0	1,732.3
Equity Securities
Fixed Maturity Securities
Level 1 from Level 2	0		0
Level 2 from Level 1	$ 7.4		$ 0