Investments Length of Time Fixed Maturity Securities had been in a Gross Unrealized Loss Position (Detail) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
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	$ 0.0	$ 68.8
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0.0	1.7
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	2.1	212.5
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0.0	9.2
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	6.0	183.2
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0.3	2.1
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	0.0	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
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	10.6	58.4
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0.4	3.8
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	11.7	12.0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	1.2	1.0
Public Utilities
Debt Securities, Available-for-sale, Unrealized Loss Position
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	92.0	740.1
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	2.1	31.3
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	105.3	325.7
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	16.1	32.1
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	26.4	81.5
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0.1	1.2
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	44.2	201.6
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0.4	7.1
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	438.0	9,240.2
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	20.6	462.2
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	1,652.1	1,704.9
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	107.2	202.2
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		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
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	573.0	10,391.0
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	23.5	502.5
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	1,815.4	2,521.7
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	$ 124.9	$ 253.9