Length of Time Fixed Maturity Securities had been in a Gross Unrealized Loss Position (Detail) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
United States Government and Government Agencies and Authorities
Debt Securities, Available-for-sale
Fair Value of Securities in Continuous Unrealized Loss Possition for Less Than 12 Months	$ 296.0	$ 157.9
Available-for-sale Securities, Continuous Unrealized Loss Position of Less than 12 Months, Accumulated Loss	10.3	3.0
Fair Value of Securities in Continuous Unrealized Loss Possition for Greater Than 12 Months	57.0	58.8
Available-for-sale Securities, Continuous Unrealized Loss Position of 12 Months or Longer, Accumulated Loss	3.7	1.8
States, Municipalities, and Political Subdivisions
Debt Securities, Available-for-sale
Fair Value of Securities in Continuous Unrealized Loss Possition for Less Than 12 Months	136.0	45.7
Available-for-sale Securities, Continuous Unrealized Loss Position of Less than 12 Months, Accumulated Loss	2.7	0.5
Fair Value of Securities in Continuous Unrealized Loss Possition for Greater Than 12 Months	23.9	22.3
Available-for-sale Securities, Continuous Unrealized Loss Position of 12 Months or Longer, Accumulated Loss	1.3	0.7
Foreign Governments
Debt Securities, Available-for-sale
Fair Value of Securities in Continuous Unrealized Loss Possition for Less Than 12 Months	59.6	13.2
Available-for-sale Securities, Continuous Unrealized Loss Position of Less than 12 Months, Accumulated Loss	2.2	0.4
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
Debt Securities, Available-for-sale
Fair Value of Securities in Continuous Unrealized Loss Possition for Less Than 12 Months	608.1	213.2
Available-for-sale Securities, Continuous Unrealized Loss Position of Less than 12 Months, Accumulated Loss	31.4	7.9
Fair Value of Securities in Continuous Unrealized Loss Possition for Greater Than 12 Months	149.6	133.5
Available-for-sale Securities, Continuous Unrealized Loss Position of 12 Months or Longer, Accumulated Loss	11.2	4.7
Mortgage/Asset-backed Securities
Debt Securities, Available-for-sale
Fair Value of Securities in Continuous Unrealized Loss Possition for Less Than 12 Months	350.3	252.5
Available-for-sale Securities, Continuous Unrealized Loss Position of Less than 12 Months, Accumulated Loss	10.3	1.4
Fair Value of Securities in Continuous Unrealized Loss Possition for Greater Than 12 Months	98.8	144.7
Available-for-sale Securities, Continuous Unrealized Loss Position of 12 Months or Longer, Accumulated Loss	5.3	3.3
All Other Corporate Bonds
Debt Securities, Available-for-sale
Fair Value of Securities in Continuous Unrealized Loss Possition for Less Than 12 Months	8,031.1	1,355.1
Available-for-sale Securities, Continuous Unrealized Loss Position of Less than 12 Months, Accumulated Loss	296.2	26.8
Fair Value of Securities in Continuous Unrealized Loss Possition for Greater Than 12 Months	724.3	785.2
Available-for-sale Securities, Continuous Unrealized Loss Position of 12 Months or Longer, Accumulated Loss	80.3	73.0
Redeemable Preferred Stocks
Debt Securities, Available-for-sale
Fair Value of Securities in Continuous Unrealized Loss Possition for Less Than 12 Months	10.8
Available-for-sale Securities, Continuous Unrealized Loss Position of Less than 12 Months, Accumulated Loss	0.2
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
Fixed Maturity Securities
Debt Securities, Available-for-sale
Fair Value of Securities in Continuous Unrealized Loss Possition for Less Than 12 Months	9,491.9	2,037.6
Available-for-sale Securities, Continuous Unrealized Loss Position of Less than 12 Months, Accumulated Loss	353.3	40.0
Fair Value of Securities in Continuous Unrealized Loss Possition for Greater Than 12 Months	1,053.6	1,144.5
Available-for-sale Securities, Continuous Unrealized Loss Position of 12 Months or Longer, Accumulated Loss	$ 101.8	$ 83.5