Transfers of Assets between Level 1 and Level 2 (Detail) - USD ($) $ in Millions	Sep. 30, 2015	Sep. 30, 2014
Three Months Ended September 30
Transfers Between Level 1 and Level 2
Level 1 from Level 2	$ 2,285.4	$ 3,826.1
Level 2 from Level 1	3,165.4	2,399.8
Three Months Ended September 30 | United States Government and Government Agencies and Authorities
Transfers Between Level 1 and Level 2
Level 1 from Level 2	0.0	169.6
Level 2 from Level 1	0.0	0.0
Three Months Ended September 30 | Public Utilities
Transfers Between Level 1 and Level 2
Level 1 from Level 2	157.2	415.1
Level 2 from Level 1	231.8	218.8
Three Months Ended September 30 | All Other Corporate Bonds
Transfers Between Level 1 and Level 2
Level 1 from Level 2	2,128.2	3,241.4
Level 2 from Level 1	2,933.6	2,181.0
Nine Months Ended September 30
Transfers Between Level 1 and Level 2
Level 1 from Level 2	4,191.1	4,736.0
Level 2 from Level 1	1,213.9	636.4
Nine Months Ended September 30 | United States Government and Government Agencies and Authorities
Transfers Between Level 1 and Level 2
Level 1 from Level 2	219.7	163.2
Level 2 from Level 1	0.0	0.0
Nine Months Ended September 30 | Public Utilities
Transfers Between Level 1 and Level 2
Level 1 from Level 2	306.1	470.8
Level 2 from Level 1	57.0	88.1
Nine Months Ended September 30 | All Other Corporate Bonds
Transfers Between Level 1 and Level 2
Level 1 from Level 2	3,665.3	4,102.0
Level 2 from Level 1	$ 1,156.9	$ 548.3