Length of Time Fixed Maturity Securities had been in a Gross Unrealized Loss Position (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities
Fair Value of Securities in Continuous Unrealized Loss Possition for Less Than 12 Months	$ 3,028.3	$ 5,164.7
Aggregate Loss Position of Securities in Unrealized Loss Position for Less Than 12 Months	104.3	264.1
Fair Value of Securities in Continuous Unrealized Loss Possition for Greater Than 12 Months	370.4	311.0
Aggregate Loss Position of Securities in Unrealized Loss Position for Greater Than 12 Months	35.5	39.8
United States Government and Government Agencies and Authorities
Schedule of Available-for-sale Securities
Fair Value of Securities in Continuous Unrealized Loss Possition for Less Than 12 Months	9.2	41.1
Aggregate Loss Position of Securities in Unrealized Loss Position for Less Than 12 Months	0.5	3.1
Fair Value of Securities in Continuous Unrealized Loss Possition for Greater Than 12 Months	5.8	5.2
Aggregate Loss Position of Securities in Unrealized Loss Position for Greater Than 12 Months	2.0	2.5
States, Municipalities, and Political Subdivisions
Schedule of Available-for-sale Securities
Fair Value of Securities in Continuous Unrealized Loss Possition for Less Than 12 Months	186.3	412.5
Aggregate Loss Position of Securities in Unrealized Loss Position for Less Than 12 Months	9.8	33.5
Fair Value of Securities in Continuous Unrealized Loss Possition for Greater Than 12 Months	55.9	37.2
Aggregate Loss Position of Securities in Unrealized Loss Position for Greater Than 12 Months	7.9	6.5
Foreign Governments
Schedule of Available-for-sale Securities
Fair Value of Securities in Continuous Unrealized Loss Possition for Less Than 12 Months	34.8	87.2
Aggregate Loss Position of Securities in Unrealized Loss Position for Less Than 12 Months	1.0	2.8
Fair Value of Securities in Continuous Unrealized Loss Possition for Greater Than 12 Months	0	0
Aggregate Loss Position of Securities in Unrealized Loss Position for Greater Than 12 Months	0	0
Public Utilities
Schedule of Available-for-sale Securities
Fair Value of Securities in Continuous Unrealized Loss Possition for Less Than 12 Months	628.0	870.6
Aggregate Loss Position of Securities in Unrealized Loss Position for Less Than 12 Months	24.2	47.1
Fair Value of Securities in Continuous Unrealized Loss Possition for Greater Than 12 Months	49.2	58.7
Aggregate Loss Position of Securities in Unrealized Loss Position for Greater Than 12 Months	3.8	5.7
Mortgage/Asset-backed Securities
Schedule of Available-for-sale Securities
Fair Value of Securities in Continuous Unrealized Loss Possition for Less Than 12 Months	261.4	341.0
Aggregate Loss Position of Securities in Unrealized Loss Position for Less Than 12 Months	2.2	3.6
Fair Value of Securities in Continuous Unrealized Loss Possition for Greater Than 12 Months	2.4	2.5
Aggregate Loss Position of Securities in Unrealized Loss Position for Greater Than 12 Months	0.4	0.4
All Other Corporate Bonds
Schedule of Available-for-sale Securities
Fair Value of Securities in Continuous Unrealized Loss Possition for Less Than 12 Months	1,908.6	3,412.3
Aggregate Loss Position of Securities in Unrealized Loss Position for Less Than 12 Months	66.6	174.0
Fair Value of Securities in Continuous Unrealized Loss Possition for Greater Than 12 Months	257.1	207.4
Aggregate Loss Position of Securities in Unrealized Loss Position for Greater Than 12 Months	$ 21.4	$ 24.7