Length of Time Fixed Maturity Securities had been in a Gross Unrealized Loss Position (Detail) (USD $) In Millions, unless otherwise specified	9 Months Ended	12 Months Ended
	Sep. 30, 2014	Dec. 31, 2013
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value of Securities in Continuous Unrealized Loss Possition for Less Than 12 Months	$ 1,568.5	$ 5,164.7
Fair Value of Securities in Continuous Unrealized Loss Possition for Greater Than 12 Months	1,117.0	311.0
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss [Abstract]
Aggregate Loss Position of Securities in Unrealized Loss Position for Less Than 12 Months	32.0	264.1
Aggregate Loss Position of Securities in Unrealized Loss Position for Greater Than 12 Months	52.6	39.8
United States Government and Government Agencies and Authorities
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value of Securities in Continuous Unrealized Loss Possition for Less Than 12 Months	0	41.1
Fair Value of Securities in Continuous Unrealized Loss Possition for Greater Than 12 Months	6.5	5.2
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss [Abstract]
Aggregate Loss Position of Securities in Unrealized Loss Position for Less Than 12 Months	0	3.1
Aggregate Loss Position of Securities in Unrealized Loss Position for Greater Than 12 Months	1.4	2.5
States, Municipalities, and Political Subdivisions
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value of Securities in Continuous Unrealized Loss Possition for Less Than 12 Months	58.5	412.5
Fair Value of Securities in Continuous Unrealized Loss Possition for Greater Than 12 Months	70.8	37.2
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss [Abstract]
Aggregate Loss Position of Securities in Unrealized Loss Position for Less Than 12 Months	0.7	33.5
Aggregate Loss Position of Securities in Unrealized Loss Position for Greater Than 12 Months	3.1	6.5
Foreign Governments
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value of Securities in Continuous Unrealized Loss Possition for Less Than 12 Months		87.2
Fair Value of Securities in Continuous Unrealized Loss Possition for Greater Than 12 Months		0
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss [Abstract]
Aggregate Loss Position of Securities in Unrealized Loss Position for Less Than 12 Months		2.8
Aggregate Loss Position of Securities in Unrealized Loss Position for Greater Than 12 Months		0
Public Utilities
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value of Securities in Continuous Unrealized Loss Possition for Less Than 12 Months	28.6	506.0
Fair Value of Securities in Continuous Unrealized Loss Possition for Greater Than 12 Months	114.7	27.5
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss [Abstract]
Aggregate Loss Position of Securities in Unrealized Loss Position for Less Than 12 Months	0.4	23.7
Aggregate Loss Position of Securities in Unrealized Loss Position for Greater Than 12 Months	4.2	2.0
Mortgage/Asset-backed Securities
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value of Securities in Continuous Unrealized Loss Possition for Less Than 12 Months	8.0	341.0
Fair Value of Securities in Continuous Unrealized Loss Possition for Greater Than 12 Months	17.4	2.5
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss [Abstract]
Aggregate Loss Position of Securities in Unrealized Loss Position for Less Than 12 Months	0	3.6
Aggregate Loss Position of Securities in Unrealized Loss Position for Greater Than 12 Months	0.6	0.4
All Other Corporate Bonds
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value of Securities in Continuous Unrealized Loss Possition for Less Than 12 Months	1,473.4	3,776.9
Fair Value of Securities in Continuous Unrealized Loss Possition for Greater Than 12 Months	907.6	238.6
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss [Abstract]
Aggregate Loss Position of Securities in Unrealized Loss Position for Less Than 12 Months	30.9	197.4
Aggregate Loss Position of Securities in Unrealized Loss Position for Greater Than 12 Months	$ 43.3	$ 28.4