Transfers of Assets between Level 1 and Level 2 (Detail) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Transfers Between Level 1 and Level 2
Level 1 from Level 2 Transfers	$ 1,376.5	$ 1,837.1
Level 2 from Level 1 Transfers	1,837.1	1,851.7
United States Government and Government Agencies and Authorities
Transfers Between Level 1 and Level 2
Level 1 from Level 2 Transfers	0.0	163.2
Level 2 from Level 1 Transfers	195.3	0.0
Public Utilities
Transfers Between Level 1 and Level 2
Level 1 from Level 2 Transfers	7.3	81.8
Level 2 from Level 1 Transfers	85.1	253.4
Corporate Debt Securities
Transfers Between Level 1 and Level 2
Level 1 from Level 2 Transfers	1,369.2	1,592.1
Level 2 from Level 1 Transfers	$ 1,556.7	$ 1,598.3