Transfers of Assets between Level 1 and Level 2 (Detail) - First Quarter [Member] - USD ($) $ in Millions	Mar. 31, 2017	Mar. 31, 2016
Transfers Between Level 1 and Level 2
Level 1 from Level 2	$ 7,475.4	$ 6,287.4
Level 2 from Level 1	1,312.8	829.4
United States Government and Government Agencies and Authorities
Transfers Between Level 1 and Level 2
Level 1 from Level 2	54.5	423.8
Level 2 from Level 1	180.0	0.0
Public Utilities
Transfers Between Level 1 and Level 2
Level 1 from Level 2	1,103.9	511.6
Level 2 from Level 1	57.5	43.0
All Other Corporate Bonds
Transfers Between Level 1 and Level 2
Level 1 from Level 2	6,317.0	5,352.0
Level 2 from Level 1	$ 1,075.3	$ 786.4