Length of Time Fixed Maturity Securities had been in a Gross Unrealized Loss Position (Detail) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
U.S. Government and Agencies
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	$ 110.2	$ 68.8
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0.4	1.7
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	0.0	212.5
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0.0	9.2
State and Municipal Securities
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	331.0	183.2
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	7.3	2.1
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	0.3	65.0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0.0	2.3
Foreign Governments
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	69.4	58.4
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	1.0	3.8
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	0.0	12.0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0.0	1.0
Public Utilities
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	168.3	740.1
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	2.6	31.3
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	37.0	325.7
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	5.6	32.1
Mortgage/Asset-backed Securities
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	47.0	81.5
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0.4	1.2
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	3.1	201.6
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0.0	7.1
All Other Corporate Bonds
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	579.1	9,240.2
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	29.1	462.2
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	379.8	1,704.9
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	44.8	202.2
Fixed Maturity Securities
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	1,305.0	10,391.0
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	40.8	502.5
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	420.2	2,521.7
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	$ 50.4	253.9
Redeemable Preferred Stocks
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value		18.8
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss		0.2
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