Transfers of Assets between Level 1 and Level 2 (Detail) - USD ($) $ in Millions	Jun. 30, 2018	Jun. 30, 2017
Second Quarter
Transfers Between Level 1 and Level 2
Level 1 from Level 2	$ 2,472.6	$ 2,291.5
Level 2 from Level 1	3,397.1	4,674.3
Second Quarter | United States Government and Government Agencies and Authorities
Transfers Between Level 1 and Level 2
Level 1 from Level 2	94.7	0.0
Level 2 from Level 1	0.0	152.4
Second Quarter | Public Utilities
Transfers Between Level 1 and Level 2
Level 1 from Level 2	224.7	360.3
Level 2 from Level 1	192.0	891.5
Second Quarter | All Other Corporate Bonds
Transfers Between Level 1 and Level 2
Level 1 from Level 2	2,153.2	1,931.2
Level 2 from Level 1	3,205.1	3,630.4
Second Quarter | Equity Securities
Transfers Between Level 1 and Level 2
Level 1 from Level 2	0.0	0.0
Level 2 from Level 1	0.0	0.0
First and Second Quarters
Transfers Between Level 1 and Level 2
Level 1 from Level 2	5,004.3	5,440.1
Level 2 from Level 1	1,265.1	1,669.9
First and Second Quarters | United States Government and Government Agencies and Authorities
Transfers Between Level 1 and Level 2
Level 1 from Level 2	40.1	0.0
Level 2 from Level 1	0.0	276.6
First and Second Quarters | Public Utilities
Transfers Between Level 1 and Level 2
Level 1 from Level 2	510.6	596.8
Level 2 from Level 1	54.1	98.3
First and Second Quarters | All Other Corporate Bonds
Transfers Between Level 1 and Level 2
Level 1 from Level 2	4,453.6	4,843.3
Level 2 from Level 1	1,211.0	1,295.0
First and Second Quarters | Equity Securities
Transfers Between Level 1 and Level 2
Level 1 from Level 2	24.2	0.0
Level 2 from Level 1	$ 14.9	$ 0.0