Transfers of Assets between Level 1 and Level 2 (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Transfers Between Level 1 and Level 2
Level 1 from Level 2 Transfers	$ 1,837.1	$ 1,607.1
Level 2 from Level 1 Transfers	1,851.7	1,191.7
United States Government and Government Agencies and Authorities
Transfers Between Level 1 and Level 2
Level 1 from Level 2 Transfers	163.2	62.2
Level 2 from Level 1 Transfers	0	0
States, Municipalities, and Political Subdivisions
Transfers Between Level 1 and Level 2
Level 1 from Level 2 Transfers	0	0
Level 2 from Level 1 Transfers	0	53.0
Public Utilities
Transfers Between Level 1 and Level 2
Level 1 from Level 2 Transfers	81.8	248.4
Level 2 from Level 1 Transfers	253.4	20.8
Corporate Debt Securities
Transfers Between Level 1 and Level 2
Level 1 from Level 2 Transfers	1,592.1	1,296.5
Level 2 from Level 1 Transfers	$ 1,598.3	$ 1,117.9