Transfers of Assets between Level 1 and Level 2 (Detail) - USD ($) $ in Millions	Jun. 30, 2017	Jun. 30, 2016
Second Quarter
Transfers Between Level 1 and Level 2
Level 1 from Level 2	$ 2,291.5	$ 3,474.4
Level 2 from Level 1	4,674.3	3,445.4
Second Quarter | United States Government and Government Agencies and Authorities
Transfers Between Level 1 and Level 2
Level 1 from Level 2	0.0	0.0
Level 2 from Level 1	152.4	0.0
Second Quarter | States, Municipalities, and Political Subdivisions
Transfers Between Level 1 and Level 2
Level 1 from Level 2	0.0	73.4
Level 2 from Level 1	0.0	0.0
Second Quarter | Public Utilities
Transfers Between Level 1 and Level 2
Level 1 from Level 2	360.3	527.2
Level 2 from Level 1	891.5	397.7
Second Quarter | All Other Corporate Bonds
Transfers Between Level 1 and Level 2
Level 1 from Level 2	1,931.2	2,873.8
Level 2 from Level 1	3,630.4	3,047.7
First and Second Quarters
Transfers Between Level 1 and Level 2
Level 1 from Level 2	5,440.1	6,340.7
Level 2 from Level 1	1,669.9	820.7
First and Second Quarters | United States Government and Government Agencies and Authorities
Transfers Between Level 1 and Level 2
Level 1 from Level 2	0.0	423.8
Level 2 from Level 1	276.6	0.0
First and Second Quarters | States, Municipalities, and Political Subdivisions
Transfers Between Level 1 and Level 2
Level 1 from Level 2	0.0	73.3
Level 2 from Level 1	0.0	0.0
First and Second Quarters | Public Utilities
Transfers Between Level 1 and Level 2
Level 1 from Level 2	596.8	630.1
Level 2 from Level 1	98.3	34.1
First and Second Quarters | All Other Corporate Bonds
Transfers Between Level 1 and Level 2
Level 1 from Level 2	4,843.3	5,213.5
Level 2 from Level 1	$ 1,295.0	$ 786.6