Length of Time Fixed Maturity Securities had been in a Gross Unrealized Loss Position (Detail) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities
Fair Value of Securities in Continuous Unrealized Loss Possition for Less Than 12 Months	$ 1,976.7	$ 4,667.3
Available-for-sale Securities, Continuous Unrealized Loss Position of Less than 12 Months, Accumulated Loss	43.0	154.9
Fair Value of Securities in Continuous Unrealized Loss Possition for Greater Than 12 Months	660.7	801.9
Available-for-sale Securities, Continuous Unrealized Loss Position of 12 Months or Longer, Accumulated Loss	67.6	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	192.5	132.8
Available-for-sale Securities, Continuous Unrealized Loss Position of Less than 12 Months, Accumulated Loss	3.8	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	75.7	132.2
Available-for-sale Securities, Continuous Unrealized Loss Position of Less than 12 Months, Accumulated Loss	1.2	4.8
Fair Value of Securities in Continuous Unrealized Loss Possition for Greater Than 12 Months	18.0	0.0
Available-for-sale Securities, Continuous Unrealized Loss Position of 12 Months or Longer, Accumulated Loss	0.5	0.0
Foreign Governments
Schedule of Available-for-sale Securities
Fair Value of Securities in Continuous Unrealized Loss Possition for Less Than 12 Months	12.4
Available-for-sale Securities, Continuous Unrealized Loss Position of Less than 12 Months, Accumulated Loss	1.1
Fair Value of Securities in Continuous Unrealized Loss Possition for Greater Than 12 Months	0.0
Available-for-sale Securities, Continuous Unrealized Loss Position of 12 Months or Longer, Accumulated Loss	0.0
Public Utilities
Schedule of Available-for-sale Securities
Fair Value of Securities in Continuous Unrealized Loss Possition for Less Than 12 Months	181.4	260.2
Available-for-sale Securities, Continuous Unrealized Loss Position of Less than 12 Months, Accumulated Loss	4.4	15.3
Fair Value of Securities in Continuous Unrealized Loss Possition for Greater Than 12 Months	63.4	15.6
Available-for-sale Securities, Continuous Unrealized Loss Position of 12 Months or Longer, Accumulated Loss	5.1	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	367.6	513.2
Available-for-sale Securities, Continuous Unrealized Loss Position of Less than 12 Months, Accumulated Loss	3.6	9.1
Fair Value of Securities in Continuous Unrealized Loss Possition for Greater Than 12 Months	1.0	0.8
Available-for-sale Securities, Continuous Unrealized Loss Position of 12 Months or Longer, Accumulated Loss	0.0	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	1,147.1	3,621.0
Available-for-sale Securities, Continuous Unrealized Loss Position of Less than 12 Months, Accumulated Loss	28.9	122.1
Fair Value of Securities in Continuous Unrealized Loss Possition for Greater Than 12 Months	578.3	774.7
Available-for-sale Securities, Continuous Unrealized Loss Position of 12 Months or Longer, Accumulated Loss	$ 62.0	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
Available-for-sale Securities, Continuous Unrealized Loss Position of Less than 12 Months, Accumulated Loss		0.1
Fair Value of Securities in Continuous Unrealized Loss Possition for Greater Than 12 Months		10.8
Available-for-sale Securities, Continuous Unrealized Loss Position of 12 Months or Longer, Accumulated Loss		$ 0.2