Transfers of Assets between Level 1 and Level 2 (Detail) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Transfers Between Level 1 and Level 2
Level 1 from Level 2 Transfers	$ 2,154.9	$ 1,376.5
Level 2 from Level 1 Transfers	1,919.4	1,837.1
United States Government and Government Agencies and Authorities
Transfers Between Level 1 and Level 2
Level 1 from Level 2 Transfers	200.0	0.0
Level 2 from Level 1 Transfers	0.0	195.3
Public Utilities
Transfers Between Level 1 and Level 2
Level 1 from Level 2 Transfers	65.9	7.3
Level 2 from Level 1 Transfers	59.1	85.1
Corporate Debt Securities
Transfers Between Level 1 and Level 2
Level 1 from Level 2 Transfers	1,889.0	1,369.2
Level 2 from Level 1 Transfers	$ 1,860.3	$ 1,556.7