Transfers of Assets between Level 1 and Level 2 (Detail) - Three Months Ended March 31 - USD ($) $ in Millions	Mar. 31, 2016	Mar. 31, 2015
Transfers Between Level 1 and Level 2
Level 1 from Level 2	$ 6,287.4	$ 5,290.1
Level 2 from Level 1	829.4	1,238.3
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
Public Utilities
Transfers Between Level 1 and Level 2
Level 1 from Level 2	511.6	606.7
Level 2 from Level 1	43.0	48.9
All Other Corporate Bonds
Transfers Between Level 1 and Level 2
Level 1 from Level 2	5,352.0	4,463.8
Level 2 from Level 1	$ 786.4	$ 1,189.4