Transfers of Assets between Level 1 and Level 2 (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Transfers Between Level 1 and Level 2 [Line Items]
Fair Value, Assets, Level 1 to Level 2 Transfers, Amount	$ 1,607.1	$ 1,115.7
Fair Value, Assets, Level 2 to Level 1 Transfers, Amount	1,191.7	3,730.5
United States Government and Government Agencies and Authorities
Transfers Between Level 1 and Level 2 [Line Items]
Fair Value, Assets, Level 1 to Level 2 Transfers, Amount	62.2	0
Fair Value, Assets, Level 2 to Level 1 Transfers, Amount	0	224.7
States, Municipalities, and Political Subdivisions
Transfers Between Level 1 and Level 2 [Line Items]
Fair Value, Assets, Level 1 to Level 2 Transfers, Amount	0	0
Fair Value, Assets, Level 2 to Level 1 Transfers, Amount	53.0	43.8
Public Utility, Bonds [Member]
Transfers Between Level 1 and Level 2 [Line Items]
Fair Value, Assets, Level 1 to Level 2 Transfers, Amount	345.9	47.1
Fair Value, Assets, Level 2 to Level 1 Transfers, Amount	53.4	653.1
All Other Corporate Bonds
Transfers Between Level 1 and Level 2 [Line Items]
Fair Value, Assets, Level 1 to Level 2 Transfers, Amount	1,199.0	1,068.6
Fair Value, Assets, Level 2 to Level 1 Transfers, Amount	$ 1,085.3	$ 2,808.9