Length of Time Fixed Maturity Securities had been in a Gross Unrealized Loss Position (Detail) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities
Fair Value of Fixed Maturity Securities in Unrealized Loss Position Less than Twelve Months	$ 4,667.3	$ 7,403.9
Accumulated Loss of Fixed Maturity Securities in Unrealized Loss Position Less than Twelve Months	154.9	496.0
Fair Value of Fixed Maturity Securities in Unrealized Loss Position Twelve Months or Longer	801.9	577.3
Accumulated Loss of Fixed Maturity Securities in Unrealized Loss Position Twelve Months or Longer	63.7	146.3
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	132.8	213.5
Accumulated Loss of Fixed Maturity Securities in Unrealized Loss Position Less than Twelve Months	3.5	7.9
Fair Value of Fixed Maturity Securities in Unrealized Loss Position Twelve Months or Longer	0.0	0.0
Accumulated Loss of Fixed Maturity Securities in Unrealized Loss Position Twelve Months or Longer	0.0	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	132.2	112.3
Accumulated Loss of Fixed Maturity Securities in Unrealized Loss Position Less than Twelve Months	4.8	4.3
Fair Value of Fixed Maturity Securities in Unrealized Loss Position Twelve Months or Longer	0.0	3.3
Accumulated Loss of Fixed Maturity Securities in Unrealized Loss Position Twelve Months or Longer	0.0	0.3
Public Utilities
Schedule of Available-for-sale Securities
Fair Value of Fixed Maturity Securities in Unrealized Loss Position Less than Twelve Months	260.2	408.4
Accumulated Loss of Fixed Maturity Securities in Unrealized Loss Position Less than Twelve Months	15.3	14.4
Fair Value of Fixed Maturity Securities in Unrealized Loss Position Twelve Months or Longer	15.6	10.3
Accumulated Loss of Fixed Maturity Securities in Unrealized Loss Position Twelve Months or Longer	1.6	1.9
Mortgage/Asset-backed Securities
Schedule of Available-for-sale Securities
Fair Value of Fixed Maturity Securities in Unrealized Loss Position Less than Twelve Months	513.2	504.3
Accumulated Loss of Fixed Maturity Securities in Unrealized Loss Position Less than Twelve Months	9.1	4.6
Fair Value of Fixed Maturity Securities in Unrealized Loss Position Twelve Months or Longer	0.8	9.0
Accumulated Loss of Fixed Maturity Securities in Unrealized Loss Position Twelve Months or Longer	0.1	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	3,621.0	6,155.0
Accumulated Loss of Fixed Maturity Securities in Unrealized Loss Position Less than Twelve Months	122.1	464.2
Fair Value of Fixed Maturity Securities in Unrealized Loss Position Twelve Months or Longer	774.7	554.7
Accumulated Loss of Fixed Maturity Securities in Unrealized Loss Position Twelve Months or Longer	61.8	144.0
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	10.4
Accumulated Loss of Fixed Maturity Securities in Unrealized Loss Position Less than Twelve Months	0.1	0.6
Fair Value of Fixed Maturity Securities in Unrealized Loss Position Twelve Months or Longer	10.8	0.0
Accumulated Loss of Fixed Maturity Securities in Unrealized Loss Position Twelve Months or Longer	$ 0.2	$ 0.0