Length of Time Fixed Maturity Securities had been in a Gross Unrealized Loss Position (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	$ 1,700.9	$ 5,164.7
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	82.4	264.1
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	838.9	311.0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	42.4	39.8
United States Government and Government Agencies and Authorities
Schedule of Available-for-sale Securities
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	0	41.1
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0	3.1
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	7.4	5.2
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0.5	2.5
States, Municipalities, and Political Subdivisions
Schedule of Available-for-sale Securities
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	1.6	412.5
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0	33.5
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	42.0	37.2
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	1.1	6.5
Foreign Governments
Schedule of Available-for-sale Securities
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value		87.2
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss		2.8
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value		0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss		0
Public Utilities
Schedule of Available-for-sale Securities
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	5.1	506.0
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0.2	23.7
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	58.2	27.5
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0.7	2.0
Mortgage/Asset-backed Securities
Schedule of Available-for-sale Securities
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	28.0	341.0
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0	3.6
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	1.9	2.5
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0.1	0.4
All Other Corporate Bonds
Schedule of Available-for-sale Securities
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	1,666.2	3,776.9
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	82.2	197.4
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	729.4	238.6
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	$ 40.0	$ 28.4