Transfers of Assets between Level 1 and Level 2 (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Transfers Between Level 1 and Level 2 [Line Items]
Level 1 to Level 2 Transfers	$ 967.1	$ 1,213.7
Level 2 to Level 1 Transfers	3,006.8	3,025.3
United States Government and Government Agencies and Authorities
Transfers Between Level 1 and Level 2 [Line Items]
Level 1 to Level 2 Transfers	121.5	0
Level 2 to Level 1 Transfers	163.2	256.9
States, Municipalities, and Political Subdivisions
Transfers Between Level 1 and Level 2 [Line Items]
Level 1 to Level 2 Transfers	0	19.4
Level 2 to Level 1 Transfers	0	0
Public Utilities
Transfers Between Level 1 and Level 2 [Line Items]
Level 1 to Level 2 Transfers	192.8	53.5
Level 2 to Level 1 Transfers	452.1	502.1
All Other Corporate Bonds
Transfers Between Level 1 and Level 2 [Line Items]
Level 1 to Level 2 Transfers	652.8	1,140.8
Level 2 to Level 1 Transfers	$ 2,391.5	$ 2,266.3