Transfers of Assets between Level 1 and Level 2 (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Jun. 30, 2012
First and Second Quarters
Fixed Maturity Securities
Level 1 from Level 2	$ 5,116.3	$ 2,941.3
Level 2 from Level 1	824.8	2,232.8
First and Second Quarters | United States Government and Government Agencies and Authorities
Fixed Maturity Securities
Level 1 from Level 2	194.1	0
Level 2 from Level 1	0	204.2
First and Second Quarters | States, Municipalities, and Political Subdivisions
Fixed Maturity Securities
Level 1 from Level 2	0	0
Level 2 from Level 1	19.4	43.8
First and Second Quarters | Public Utilities
Fixed Maturity Securities
Level 1 from Level 2	854.4	647.6
Level 2 from Level 1	33.5	334.9
First and Second Quarters | All Other Corporate Bonds
Fixed Maturity Securities
Level 1 from Level 2	4,067.8	2,293.7
Level 2 from Level 1	771.9	1,649.9
Second Quarter
Fixed Maturity Securities
Level 1 from Level 2	3,627.5	2,492.1
Level 2 from Level 1	1,346.4	3,156.5
Second Quarter | United States Government and Government Agencies and Authorities
Fixed Maturity Securities
Level 1 from Level 2	0	15.5
Level 2 from Level 1	61.4	0
Second Quarter | Public Utilities
Fixed Maturity Securities
Level 1 from Level 2	656.0	745.1
Level 2 from Level 1	294.8	759.1
Second Quarter | All Other Corporate Bonds
Fixed Maturity Securities
Level 1 from Level 2	2,971.5	1,731.5
Level 2 from Level 1	990.2	2,397.4
Second Quarter | Equity Securites
Fixed Maturity Securities
Level 1 from Level 2	0	8.1
Level 2 from Level 1	$ 0	$ 0