Transfers of Assets between Level 1 and Level 2 (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Fixed Maturity Securities
Level 1 from Level 2	$ 2,492.1	$ 2,473.4	$ 2,941.3	$ 3,348.3
Level 2 from Level 1	3,156.5	3,109.5	2,232.8	2,357.7
United States Government and Government Agencies and Authorities
Fixed Maturity Securities
Level 1 from Level 2	15.5	0	0	16.1
Level 2 from Level 1	0	0	204.2	0
States, Municipalities, and Political Subdivisions
Fixed Maturity Securities
Level 1 from Level 2	0	0	0	25.4
Level 2 from Level 1	0	27.0	43.8	301.9
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	745.1	531.8	647.6	698.3
Level 2 from Level 1	759.1	762.1	334.9	526.9
All Other Corporate Bonds
Fixed Maturity Securities
Level 1 from Level 2	1,731.5	1,941.6	2,293.7	2,608.5
Level 2 from Level 1	2,397.4	2,320.4	1,649.9	1,528.2
Equity Securities
Fixed Maturity Securities
Level 1 from Level 2	8.1	9.9	0	9.0
Level 2 from Level 1	$ 0	$ 0	$ 0	$ 0