Length of Time Fixed Maturity Securities had been in a Gross Unrealized Loss Position (Detail) - USD ($) $ in Millions	Sep. 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	$ 5,842.1	$ 1,700.9
Available-for-sale Securities, Continuous Unrealized Loss Position of Less than 12 Months, Accumulated Loss	324.1	82.4
Fair Value of Securities in Continuous Unrealized Loss Possition for Greater Than 12 Months	426.0	838.9
Available-for-sale Securities, Continuous Unrealized Loss Position of 12 Months or Longer, Accumulated Loss	68.2	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	243.3	0.0
Available-for-sale Securities, Continuous Unrealized Loss Position of Less than 12 Months, Accumulated Loss	3.0	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	170.1	1.6
Available-for-sale Securities, Continuous Unrealized Loss Position of Less than 12 Months, Accumulated Loss	4.8	0.0
Fair Value of Securities in Continuous Unrealized Loss Possition for Greater Than 12 Months	3.3	42.0
Available-for-sale Securities, Continuous Unrealized Loss Position of 12 Months or Longer, Accumulated Loss	0.4	1.1
Public Utilities
Schedule of Available-for-sale Securities
Fair Value of Securities in Continuous Unrealized Loss Possition for Less Than 12 Months	260.8	5.1
Available-for-sale Securities, Continuous Unrealized Loss Position of Less than 12 Months, Accumulated Loss	9.0	0.2
Fair Value of Securities in Continuous Unrealized Loss Possition for Greater Than 12 Months	6.1	58.2
Available-for-sale Securities, Continuous Unrealized Loss Position of 12 Months or Longer, Accumulated Loss	0.9	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	104.3	28.0
Available-for-sale Securities, Continuous Unrealized Loss Position of Less than 12 Months, Accumulated Loss	0.9	0.0
Fair Value of Securities in Continuous Unrealized Loss Possition for Greater Than 12 Months	9.0	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	5,052.8	1,666.2
Available-for-sale Securities, Continuous Unrealized Loss Position of Less than 12 Months, Accumulated Loss	306.2	82.2
Fair Value of Securities in Continuous Unrealized Loss Possition for Greater Than 12 Months	407.6	729.4
Available-for-sale Securities, Continuous Unrealized Loss Position of 12 Months or Longer, Accumulated Loss	66.8	$ 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