Transfers of Assets between Level 1 and Level 2 (Detail) - USD ($) $ in Millions	Jun. 30, 2016	Jun. 30, 2015
First and Second Quarters [Member]
Transfers Between Level 1 and Level 2
Level 1 from Level 2	$ 6,340.7	$ 5,164.0
Level 2 from Level 1	820.7	1,074.6
First and Second Quarters [Member] | United States Government and Government Agencies and Authorities
Transfers Between Level 1 and Level 2
Level 1 from Level 2	423.8	219.6
Level 2 from Level 1	0.0	0.0
First and Second Quarters [Member] | States, Municipalities, and Political Subdivisions
Transfers Between Level 1 and Level 2
Level 1 from Level 2	73.3	0.0
Level 2 from Level 1	0.0	0.0
First and Second Quarters [Member] | Public Utilities
Transfers Between Level 1 and Level 2
Level 1 from Level 2	630.1	385.1
Level 2 from Level 1	34.1	56.9
First and Second Quarters [Member] | All Other Corporate Bonds
Transfers Between Level 1 and Level 2
Level 1 from Level 2	5,213.5	4,559.3
Level 2 from Level 1	786.6	1,017.7
Second Quarter [Member]
Transfers Between Level 1 and Level 2
Level 1 from Level 2	3,474.4	3,020.0
Level 2 from Level 1	3,445.4	3,189.7
Second Quarter [Member] | States, Municipalities, and Political Subdivisions
Transfers Between Level 1 and Level 2
Level 1 from Level 2	73.4	0.0
Level 2 from Level 1	0.0	0.0
Second Quarter [Member] | Public Utilities
Transfers Between Level 1 and Level 2
Level 1 from Level 2	527.2	241.2
Level 2 from Level 1	397.7	453.3
Second Quarter [Member] | All Other Corporate Bonds
Transfers Between Level 1 and Level 2
Level 1 from Level 2	2,873.8	2,778.8
Level 2 from Level 1	$ 3,047.7	$ 2,736.4