Length of Time Fixed Maturity Securities had been in a Gross Unrealized Loss Position (Detail) - USD ($) $ in Millions	Sep. 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	$ 390.6	$ 157.9
Available-for-sale Securities, Continuous Unrealized Loss Position of Less than 12 Months, Accumulated Loss	10.4	3.0
Fair Value of Securities in Continuous Unrealized Loss Possition for Greater Than 12 Months	181.3	58.8
Available-for-sale Securities, Continuous Unrealized Loss Position of 12 Months or Longer, Accumulated Loss	15.2	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	359.3	45.7
Available-for-sale Securities, Continuous Unrealized Loss Position of Less than 12 Months, Accumulated Loss	7.5	0.5
Fair Value of Securities in Continuous Unrealized Loss Possition for Greater Than 12 Months	34.1	22.3
Available-for-sale Securities, Continuous Unrealized Loss Position of 12 Months or Longer, Accumulated Loss	2.2	0.7
Foreign Governments
Debt Securities, Available-for-sale
Fair Value of Securities in Continuous Unrealized Loss Possition for Less Than 12 Months	34.4	13.2
Available-for-sale Securities, Continuous Unrealized Loss Position of Less than 12 Months, Accumulated Loss	0.7	0.4
Fair Value of Securities in Continuous Unrealized Loss Possition for Greater Than 12 Months	12.3	0.0
Available-for-sale Securities, Continuous Unrealized Loss Position of 12 Months or Longer, Accumulated Loss	1.0	0.0
Public Utilities
Debt Securities, Available-for-sale
Fair Value of Securities in Continuous Unrealized Loss Possition for Less Than 12 Months	633.4	213.2
Available-for-sale Securities, Continuous Unrealized Loss Position of Less than 12 Months, Accumulated Loss	33.6	7.9
Fair Value of Securities in Continuous Unrealized Loss Possition for Greater Than 12 Months	228.4	133.5
Available-for-sale Securities, Continuous Unrealized Loss Position of 12 Months or Longer, Accumulated Loss	19.8	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	515.6	252.5
Available-for-sale Securities, Continuous Unrealized Loss Position of Less than 12 Months, Accumulated Loss	9.1	1.4
Fair Value of Securities in Continuous Unrealized Loss Possition for Greater Than 12 Months	212.7	144.7
Available-for-sale Securities, Continuous Unrealized Loss Position of 12 Months or Longer, Accumulated Loss	12.0	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	7,839.8	1,355.1
Available-for-sale Securities, Continuous Unrealized Loss Position of Less than 12 Months, Accumulated Loss	282.6	26.8
Fair Value of Securities in Continuous Unrealized Loss Possition for Greater Than 12 Months	1,164.0	785.2
Available-for-sale Securities, Continuous Unrealized Loss Position of 12 Months or Longer, Accumulated Loss	102.3	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	9,783.9	2,037.6
Available-for-sale Securities, Continuous Unrealized Loss Position of Less than 12 Months, Accumulated Loss	344.1	40.0
Fair Value of Securities in Continuous Unrealized Loss Possition for Greater Than 12 Months	1,832.8	1,144.5
Available-for-sale Securities, Continuous Unrealized Loss Position of 12 Months or Longer, Accumulated Loss	$ 152.5	$ 83.5