Length of Time Fixed Maturity Securities had been in a Gross Unrealized Loss Position (Detail) - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities
Fair Value of Securities in Continuous Unrealized Loss Possition for Less Than 12 Months	$ 5,441.8	$ 1,700.9
Available-for-sale Securities, Continuous Unrealized Loss Position of Less than 12 Months, Accumulated Loss	186.4	82.4
Fair Value of Securities in Continuous Unrealized Loss Possition for Greater Than 12 Months	313.7	838.9
Available-for-sale Securities, Continuous Unrealized Loss Position of 12 Months or Longer, Accumulated Loss	29.5	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	246.5	0.0
Available-for-sale Securities, Continuous Unrealized Loss Position of Less than 12 Months, Accumulated Loss	10.0	0.0
Fair Value of Securities in Continuous Unrealized Loss Possition for Greater Than 12 Months	0.0	7.4
Available-for-sale Securities, Continuous Unrealized Loss Position of 12 Months or Longer, Accumulated Loss	0.0	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	228.1	1.6
Available-for-sale Securities, Continuous Unrealized Loss Position of Less than 12 Months, Accumulated Loss	6.1	0.0
Fair Value of Securities in Continuous Unrealized Loss Possition for Greater Than 12 Months	1.8	42.0
Available-for-sale Securities, 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	262.3	5.1
Available-for-sale Securities, Continuous Unrealized Loss Position of Less than 12 Months, Accumulated Loss	6.5	0.2
Fair Value of Securities in Continuous Unrealized Loss Possition for Greater Than 12 Months	6.7	58.2
Available-for-sale Securities, Continuous Unrealized Loss Position of 12 Months or Longer, Accumulated Loss	0.4	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	364.4	28.0
Available-for-sale Securities, Continuous Unrealized Loss Position of Less than 12 Months, Accumulated Loss	5.4	0.0
Fair Value of Securities in Continuous Unrealized Loss Possition for Greater Than 12 Months	1.0	1.9
Available-for-sale Securities, 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	4,329.7	1,666.2
Available-for-sale Securities, Continuous Unrealized Loss Position of Less than 12 Months, Accumulated Loss	158.2	82.2
Fair Value of Securities in Continuous Unrealized Loss Possition for Greater Than 12 Months	304.2	729.4
Available-for-sale Securities, Continuous Unrealized Loss Position of 12 Months or Longer, Accumulated Loss	28.8	$ 40.0
Redeemable Preferred Stocks
Schedule of Available-for-sale Securities
Fair Value of Securities in Continuous Unrealized Loss Possition for Less Than 12 Months	10.8
Available-for-sale Securities, Continuous Unrealized Loss Position of Less than 12 Months, Accumulated Loss	0.2
Fair Value of Securities in Continuous Unrealized Loss Possition for Greater Than 12 Months	0.0
Available-for-sale Securities, Continuous Unrealized Loss Position of 12 Months or Longer, Accumulated Loss	$ 0.0