Length of Time Fixed Maturity Securities had been in a Gross Unrealized Loss Position (Detail) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities
Fair Value of Securities in Continuous Unrealized Loss Possition for Less Than 12 Months	$ 3,848.2	$ 4,667.3
Available-for-sale Securities, Continuous Unrealized Loss Position of Less than 12 Months, Accumulated Loss	101.3	154.9
Fair Value of Securities in Continuous Unrealized Loss Possition for Greater Than 12 Months	724.1	801.9
Available-for-sale Securities, Continuous Unrealized Loss Position of 12 Months or Longer, Accumulated Loss	60.3	63.7
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	157.8	132.8
Available-for-sale Securities, Continuous Unrealized Loss Position of Less than 12 Months, Accumulated Loss	3.6	3.5
Fair Value of Securities in Continuous Unrealized Loss Possition for Greater Than 12 Months	0.0	0.0
Available-for-sale Securities, Continuous Unrealized Loss Position of 12 Months or Longer, Accumulated Loss	0.0	0.0
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	111.1	132.2
Available-for-sale Securities, Continuous Unrealized Loss Position of Less than 12 Months, Accumulated Loss	3.1	4.8
Fair Value of Securities in Continuous Unrealized Loss Possition for Greater Than 12 Months	0.0	0.0
Available-for-sale Securities, Continuous Unrealized Loss Position of 12 Months or Longer, Accumulated Loss	0.0	0.0
Public Utilities
Schedule of Available-for-sale Securities
Fair Value of Securities in Continuous Unrealized Loss Possition for Less Than 12 Months	260.5	260.2
Available-for-sale Securities, Continuous Unrealized Loss Position of Less than 12 Months, Accumulated Loss	9.9	15.3
Fair Value of Securities in Continuous Unrealized Loss Possition for Greater Than 12 Months	31.5	15.6
Available-for-sale Securities, Continuous Unrealized Loss Position of 12 Months or Longer, Accumulated Loss	5.0	1.6
Mortgage/Asset-backed Securities
Schedule of Available-for-sale Securities
Fair Value of Securities in Continuous Unrealized Loss Possition for Less Than 12 Months	499.8	513.2
Available-for-sale Securities, Continuous Unrealized Loss Position of Less than 12 Months, Accumulated Loss	9.7	9.1
Fair Value of Securities in Continuous Unrealized Loss Possition for Greater Than 12 Months	9.3	0.8
Available-for-sale Securities, Continuous Unrealized Loss Position of 12 Months or Longer, Accumulated Loss	0.4	0.1
All Other Corporate Bonds
Schedule of Available-for-sale Securities
Fair Value of Securities in Continuous Unrealized Loss Possition for Less Than 12 Months	2,811.1	3,621.0
Available-for-sale Securities, Continuous Unrealized Loss Position of Less than 12 Months, Accumulated Loss	74.9	122.1
Fair Value of Securities in Continuous Unrealized Loss Possition for Greater Than 12 Months	672.4	774.7
Available-for-sale Securities, Continuous Unrealized Loss Position of 12 Months or Longer, Accumulated Loss	54.8	61.8
Redeemable Preferred Stocks
Schedule of Available-for-sale Securities
Fair Value of Securities in Continuous Unrealized Loss Possition for Less Than 12 Months	7.9	7.9
Available-for-sale Securities, Continuous Unrealized Loss Position of Less than 12 Months, Accumulated Loss	0.1	0.1
Fair Value of Securities in Continuous Unrealized Loss Possition for Greater Than 12 Months	10.9	10.8
Available-for-sale Securities, Continuous Unrealized Loss Position of 12 Months or Longer, Accumulated Loss	$ 0.1	$ 0.2