Length of Time Fixed Maturity Securities had been in a Gross Unrealized Loss Position (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities
Fair Value of Securities in Continuous Unrealized Loss Possition for Less Than 12 Months	$ 1,174.6	$ 1,700.9
Accumulated Loss for Securities with Continuous Unrealized Loss Position of Less than 12 Months, Accumulated Loss	68.4	82.4
Fair Value of Securities in Continuous Unrealized Loss Possition for Greater Than 12 Months	343.4	838.9
Accumulated Loss for Securities with Continuous Unrealized Loss Position of 12 Months or Longer, Accumulated Loss	21.5	42.4
United States Government and Government Agencies and Authorities
Schedule of Available-for-sale Securities
Fair Value of Securities in Continuous Unrealized Loss Possition for Less Than 12 Months	20.7	0
Accumulated Loss for Securities with Continuous Unrealized Loss Position of Less than 12 Months, Accumulated Loss	0.2	0
Fair Value of Securities in Continuous Unrealized Loss Possition for Greater Than 12 Months	0	7.4
Accumulated Loss for Securities with Continuous Unrealized Loss Position of 12 Months or Longer, Accumulated Loss	0	0.5
States, Municipalities, and Political Subdivisions
Schedule of Available-for-sale Securities
Fair Value of Securities in Continuous Unrealized Loss Possition for Less Than 12 Months	1.6	1.6
Accumulated Loss for Securities with Continuous Unrealized Loss Position of Less than 12 Months, Accumulated Loss	0	0
Fair Value of Securities in Continuous Unrealized Loss Possition for Greater Than 12 Months	1.8	42.0
Accumulated Loss for Securities with Continuous Unrealized Loss Position of 12 Months or Longer, Accumulated Loss	0.2	1.1
Public Utilities
Schedule of Available-for-sale Securities
Fair Value of Securities in Continuous Unrealized Loss Possition for Less Than 12 Months	24.8	5.1
Accumulated Loss for Securities with Continuous Unrealized Loss Position of Less than 12 Months, Accumulated Loss	0.3	0.2
Fair Value of Securities in Continuous Unrealized Loss Possition for Greater Than 12 Months	6.8	58.2
Accumulated Loss for Securities with Continuous Unrealized Loss Position of 12 Months or Longer, Accumulated Loss	0.2	0.7
Mortgage/Asset-backed Securities
Schedule of Available-for-sale Securities
Fair Value of Securities in Continuous Unrealized Loss Possition for Less Than 12 Months	19.0	28.0
Accumulated Loss for Securities with Continuous Unrealized Loss Position of Less than 12 Months, Accumulated Loss	0	0
Fair Value of Securities in Continuous Unrealized Loss Possition for Greater Than 12 Months	1.9	1.9
Accumulated Loss for Securities with Continuous Unrealized Loss Position of 12 Months or Longer, Accumulated Loss	0.1	0.1
All Other Corporate Bonds
Schedule of Available-for-sale Securities
Fair Value of Securities in Continuous Unrealized Loss Possition for Less Than 12 Months	1,108.5	1,666.2
Accumulated Loss for Securities with Continuous Unrealized Loss Position of Less than 12 Months, Accumulated Loss	67.9	82.2
Fair Value of Securities in Continuous Unrealized Loss Possition for Greater Than 12 Months	332.9	729.4
Accumulated Loss for Securities with Continuous Unrealized Loss Position of 12 Months or Longer, Accumulated Loss	$ 21.0	$ 40.0