Length of Time Fixed Maturity Securities had been in a Gross Unrealized Loss Position (Detail) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
United States Government and Government Agencies and Authorities
Debt Securities, Available-for-sale, Unrealized Loss Position
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	$ 5.9
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0.1
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	0.0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0.0
States, Municipalities, and Political Subdivisions
Debt Securities, Available-for-sale, Unrealized Loss Position
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	888.1	$ 133.4
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	38.7	1.5
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	0.6	0.1
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0.0	0.0
Foreign Governments
Debt Securities, Available-for-sale, Unrealized Loss Position
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	61.8	20.3
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	2.7	1.2
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	0.0	0.0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0.0	0.0
Public Utilities
Debt Securities, Available-for-sale, Unrealized Loss Position
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	321.4	76.3
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	22.8	3.7
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	38.6	25.4
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	3.3	2.3
Mortgage/Asset-backed Securities
Debt Securities, Available-for-sale, Unrealized Loss Position
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	0.6	3.0
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0.1	0.1
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	3.1	3.1
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0.0	0.1
All Other Corporate Bonds
Debt Securities, Available-for-sale, Unrealized Loss Position
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	1,883.4	520.4
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	84.0	22.4
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	246.0	113.5
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	26.0	24.5
Redeemable Preferred Stocks
Debt Securities, Available-for-sale, Unrealized Loss Position
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value		9.5
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss		0.1
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value		0.0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss		0.0
Fixed Maturity Securities
Debt Securities, Available-for-sale, Unrealized Loss Position
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	3,161.2	762.9
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	148.4	29.0
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	288.3	142.1
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	$ 29.3	$ 26.9