Length of Time Fixed Maturity Securities had been in a Gross Unrealized Loss Position (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities
Fair Value of Fixed Maturity Securities in Unrealized Loss Position Less than Twelve Months	$ 2,037.6	$ 4,667.3
Accumulated Loss of Fixed Maturity Securities in Unrealized Loss Position Less than Twelve Months	40.0	154.9
Fair Value of Fixed Maturity Securities in Unrealized Loss Position Twelve Months or Longer	1,144.5	801.9
Accumulated Loss of Fixed Maturity Securities in Unrealized Loss Position Twelve Months or Longer	83.5	63.7
United States Government and Government Agencies and Authorities
Schedule of Available-for-sale Securities
Fair Value of Fixed Maturity Securities in Unrealized Loss Position Less than Twelve Months	157.9	132.8
Accumulated Loss of Fixed Maturity Securities in Unrealized Loss Position Less than Twelve Months	3.0	3.5
Fair Value of Fixed Maturity Securities in Unrealized Loss Position Twelve Months or Longer	58.8	0.0
Accumulated Loss of Fixed Maturity Securities in Unrealized Loss Position Twelve Months or Longer	1.8	0.0
States, Municipalities, and Political Subdivisions
Schedule of Available-for-sale Securities
Fair Value of Fixed Maturity Securities in Unrealized Loss Position Less than Twelve Months	45.7	132.2
Accumulated Loss of Fixed Maturity Securities in Unrealized Loss Position Less than Twelve Months	0.5	4.8
Fair Value of Fixed Maturity Securities in Unrealized Loss Position Twelve Months or Longer	22.3	0.0
Accumulated Loss of Fixed Maturity Securities in Unrealized Loss Position Twelve Months or Longer	0.7	0.0
Foreign Governments
Schedule of Available-for-sale Securities
Fair Value of Fixed Maturity Securities in Unrealized Loss Position Less than Twelve Months	13.2
Accumulated Loss of Fixed Maturity Securities in Unrealized Loss Position Less than Twelve Months	0.4
Fair Value of Fixed Maturity Securities in Unrealized Loss Position Twelve Months or Longer	0.0
Accumulated Loss of Fixed Maturity Securities in Unrealized Loss Position Twelve Months or Longer	0.0
Public Utilities
Schedule of Available-for-sale Securities
Fair Value of Fixed Maturity Securities in Unrealized Loss Position Less than Twelve Months	213.2	260.2
Accumulated Loss of Fixed Maturity Securities in Unrealized Loss Position Less than Twelve Months	7.9	15.3
Fair Value of Fixed Maturity Securities in Unrealized Loss Position Twelve Months or Longer	133.5	15.6
Accumulated Loss of Fixed Maturity Securities in Unrealized Loss Position Twelve Months or Longer	4.7	1.6
Mortgage/Asset-backed Securities
Schedule of Available-for-sale Securities
Fair Value of Fixed Maturity Securities in Unrealized Loss Position Less than Twelve Months	252.5	513.2
Accumulated Loss of Fixed Maturity Securities in Unrealized Loss Position Less than Twelve Months	1.4	9.1
Fair Value of Fixed Maturity Securities in Unrealized Loss Position Twelve Months or Longer	144.7	0.8
Accumulated Loss of Fixed Maturity Securities in Unrealized Loss Position Twelve Months or Longer	3.3	0.1
All Other Corporate Bonds
Schedule of Available-for-sale Securities
Fair Value of Fixed Maturity Securities in Unrealized Loss Position Less than Twelve Months	1,355.1	3,621.0
Accumulated Loss of Fixed Maturity Securities in Unrealized Loss Position Less than Twelve Months	26.8	122.1
Fair Value of Fixed Maturity Securities in Unrealized Loss Position Twelve Months or Longer	785.2	774.7
Accumulated Loss of Fixed Maturity Securities in Unrealized Loss Position Twelve Months or Longer	$ 73.0	61.8
Redeemable Preferred Stocks
Schedule of Available-for-sale Securities
Fair Value of Fixed Maturity Securities in Unrealized Loss Position Less than Twelve Months		7.9
Accumulated Loss of Fixed Maturity Securities in Unrealized Loss Position Less than Twelve Months		0.1
Fair Value of Fixed Maturity Securities in Unrealized Loss Position Twelve Months or Longer		10.8
Accumulated Loss of Fixed Maturity Securities in Unrealized Loss Position Twelve Months or Longer		$ 0.2