Length of Time Fixed Maturity Securities had been in a Gross Unrealized Loss Position (Detail) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities
Fair Value of Fixed Maturity Securities in Unrealized Loss Position Less than Twelve Months	$ 7,403.9	$ 1,700.9
Accumulated Loss of Fixed Maturity Securities in Unrealized Loss Position Less than Twelve Months	496.0	82.4
Fair Value of Fixed Maturity Securities in Unrealized Loss Position Twelve Months or Longer	577.3	838.9
Accumulated Loss of Fixed Maturity Securities in Unrealized Loss Position Twelve Months or Longer	146.3	42.4
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	213.5	0.0
Accumulated Loss of Fixed Maturity Securities in Unrealized Loss Position Less than Twelve Months	7.9	0.0
Fair Value of Fixed Maturity Securities in Unrealized Loss Position Twelve Months or Longer	0.0	7.4
Accumulated Loss of Fixed Maturity Securities in Unrealized Loss Position Twelve Months or Longer	0.0	0.5
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	112.3	1.6
Accumulated Loss of Fixed Maturity Securities in Unrealized Loss Position Less than Twelve Months	4.3	0.0
Fair Value of Fixed Maturity Securities in Unrealized Loss Position Twelve Months or Longer	3.3	42.0
Accumulated Loss of Fixed Maturity Securities in Unrealized Loss Position Twelve Months or Longer	0.3	1.1
Public Utilities
Schedule of Available-for-sale Securities
Fair Value of Fixed Maturity Securities in Unrealized Loss Position Less than Twelve Months	408.4	5.1
Accumulated Loss of Fixed Maturity Securities in Unrealized Loss Position Less than Twelve Months	14.4	0.2
Fair Value of Fixed Maturity Securities in Unrealized Loss Position Twelve Months or Longer	10.3	58.2
Accumulated Loss of Fixed Maturity Securities in Unrealized Loss Position Twelve Months or Longer	1.9	0.7
Mortgage/Asset-backed Securities
Schedule of Available-for-sale Securities
Fair Value of Fixed Maturity Securities in Unrealized Loss Position Less than Twelve Months	504.3	28.0
Accumulated Loss of Fixed Maturity Securities in Unrealized Loss Position Less than Twelve Months	4.6	0.0
Fair Value of Fixed Maturity Securities in Unrealized Loss Position Twelve Months or Longer	9.0	1.9
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	6,155.0	1,666.2
Accumulated Loss of Fixed Maturity Securities in Unrealized Loss Position Less than Twelve Months	464.2	82.2
Fair Value of Fixed Maturity Securities in Unrealized Loss Position Twelve Months or Longer	554.7	729.4
Accumulated Loss of Fixed Maturity Securities in Unrealized Loss Position Twelve Months or Longer	144.0	$ 40.0
Redeemable Preferred Stocks
Schedule of Available-for-sale Securities
Fair Value of Fixed Maturity Securities in Unrealized Loss Position Less than Twelve Months	10.4
Accumulated Loss of Fixed Maturity Securities in Unrealized Loss Position Less than Twelve Months	0.6
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