Length of Time Fixed Maturity Securities had been in a Gross Unrealized Loss Position (Detail) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
U.S. Government and Agencies
Debt Securities, Available-for-sale
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	$ 68.8	$ 157.9
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	1.7	3.0
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	212.5	58.8
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	9.2	1.8
State and Municipal Securities
Debt Securities, Available-for-sale
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	183.2	45.7
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	2.1	0.5
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	65.0	22.3
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	2.3	0.7
Foreign Governments
Debt Securities, Available-for-sale
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	58.4	13.2
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	3.8	0.4
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	12.0	0.0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	1.0	0.0
Public Utilities
Debt Securities, Available-for-sale
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	740.1	213.2
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	31.3	7.9
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	325.7	133.5
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	32.1	4.7
Mortgage/Asset-backed Securities
Debt Securities, Available-for-sale
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	81.5	252.5
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	1.2	1.4
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	201.6	144.7
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	7.1	3.3
All Other Corporate Bonds
Debt Securities, Available-for-sale
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	9,240.2	1,355.1
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	462.2	26.8
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	1,704.9	785.2
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	202.2	73.0
Redeemable Preferred Stocks
Debt Securities, Available-for-sale
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
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	10,391.0	2,037.6
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	502.5	40.0
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	2,521.7	1,144.5
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	$ 253.9	$ 83.5