Transfers of Assets between Level 1 and Level 2 (Detail) - USD ($) $ in Millions	Jun. 30, 2015	Jun. 30, 2014
Three Months Ended June 30
Transfers Between Level 1 and Level 2
Level 1 from Level 2	$ 3,020.0	$ 2,926.4
Level 2 from Level 1	3,189.7	2,156.6
Three Months Ended June 30 | United States Government and Government Agencies and Authorities
Transfers Between Level 1 and Level 2
Level 1 from Level 2	0.0	93.3
Level 2 from Level 1	0.0	166.0
Three Months Ended June 30 | Public Utilities
Transfers Between Level 1 and Level 2
Level 1 from Level 2	241.2	275.4
Level 2 from Level 1	453.3	265.9
Three Months Ended June 30 | All Other Corporate Bonds
Transfers Between Level 1 and Level 2
Level 1 from Level 2	2,778.8	2,557.7
Level 2 from Level 1	2,736.4	1,724.7
Six Months Ended June 30
Transfers Between Level 1 and Level 2
Level 1 from Level 2	5,164.0	3,649.9
Level 2 from Level 1	1,074.6	848.3
Six Months Ended June 30 | United States Government and Government Agencies and Authorities
Transfers Between Level 1 and Level 2
Level 1 from Level 2	219.6	0.0
Level 2 from Level 1	0.0	0.0
Six Months Ended June 30 | Public Utilities
Transfers Between Level 1 and Level 2
Level 1 from Level 2	385.1	288.7
Level 2 from Level 1	56.9	81.1
Six Months Ended June 30 | All Other Corporate Bonds
Transfers Between Level 1 and Level 2
Level 1 from Level 2	4,559.3	3,361.2
Level 2 from Level 1	$ 1,017.7	$ 767.2