Transfers of Assets between Level 1 and Level 2 (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Transfers Between Level 1 and Level 2
Level 1 from Level 2 Transfers	$ 1,872.7	$ 2,154.9
Level 2 from Level 1 Transfers	2,307.2	1,919.4
United States Government and Government Agencies and Authorities
Transfers Between Level 1 and Level 2
Level 1 from Level 2 Transfers	0.0	200.0
Level 2 from Level 1 Transfers	0.0	0.0
Public Utilities
Transfers Between Level 1 and Level 2
Level 1 from Level 2 Transfers	102.9	65.9
Level 2 from Level 1 Transfers	95.5	59.1
Corporate Debt Securities
Transfers Between Level 1 and Level 2
Level 1 from Level 2 Transfers	1,769.8	1,889.0
Level 2 from Level 1 Transfers	$ 2,211.7	$ 1,860.3