Length of Time Fixed Maturity Securities had been in a Gross Unrealized Loss Position (Detail) - USD ($) $ in Millions	Jun. 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	$ 2,388.5	$ 4,667.3
Available-for-sale Securities, Continuous Unrealized Loss Position of Less than 12 Months, Accumulated Loss	49.9	154.9
Fair Value of Securities in Continuous Unrealized Loss Possition for Greater Than 12 Months	674.3	801.9
Available-for-sale Securities, Continuous Unrealized Loss Position of 12 Months or Longer, Accumulated Loss	69.8	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	133.7	132.8
Available-for-sale Securities, Continuous Unrealized Loss Position of Less than 12 Months, Accumulated Loss	2.7	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	91.6	132.2
Available-for-sale Securities, Continuous Unrealized Loss Position of Less than 12 Months, Accumulated Loss	1.8	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	219.3	260.2
Available-for-sale Securities, Continuous Unrealized Loss Position of Less than 12 Months, Accumulated Loss	5.8	15.3
Fair Value of Securities in Continuous Unrealized Loss Possition for Greater Than 12 Months	32.8	15.6
Available-for-sale Securities, Continuous Unrealized Loss Position of 12 Months or Longer, Accumulated Loss	3.5	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	449.2	513.2
Available-for-sale Securities, Continuous Unrealized Loss Position of Less than 12 Months, Accumulated Loss	6.0	9.1
Fair Value of Securities in Continuous Unrealized Loss Possition for Greater Than 12 Months	9.9	0.8
Available-for-sale Securities, Continuous Unrealized Loss Position of 12 Months or Longer, Accumulated Loss	0.2	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	1,494.7	3,621.0
Available-for-sale Securities, Continuous Unrealized Loss Position of Less than 12 Months, Accumulated Loss	33.6	122.1
Fair Value of Securities in Continuous Unrealized Loss Possition for Greater Than 12 Months	631.6	774.7
Available-for-sale Securities, Continuous Unrealized Loss Position of 12 Months or Longer, Accumulated Loss	$ 66.1	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