Length of Time Fixed Maturity Securities had been in a Gross Unrealized Loss Position (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months Fair Value	$ 1,599.0	$ 2,041.7
Less than 12 Months Gross Unrealized Loss	63.2	79.5
12 Months or Greater Fair Value	815.3	949.2
12 Months or Greater Gross Unrealized Loss	94.2	142.7
United States Government and Government Agencies and Authorities
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months Fair Value	8.9	0
Less than 12 Months Gross Unrealized Loss	0.2	0
12 Months or Greater Fair Value	5.6	6.3
12 Months or Greater Gross Unrealized Loss	1.8	1.1
States, Municipalities, and Political Subdivisions
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months Fair Value	68.7	51.6
Less than 12 Months Gross Unrealized Loss	3.8	1.3
12 Months or Greater Fair Value	74.8	75.3
12 Months or Greater Gross Unrealized Loss	7.4	7.5
Public Utilities
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months Fair Value	229.4	192.0
Less than 12 Months Gross Unrealized Loss	19.0	7.9
12 Months or Greater Fair Value	128.9	142.2
12 Months or Greater Gross Unrealized Loss	17.1	20.7
Mortgage/Asset-backed Securities
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months Fair Value	38.6	94.2
Less than 12 Months Gross Unrealized Loss	0.3	4.8
12 Months or Greater Fair Value	9.0	19.6
12 Months or Greater Gross Unrealized Loss	0.6	0.7
All Other Corporate Bonds
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months Fair Value	1,253.4	1,703.9
Less than 12 Months Gross Unrealized Loss	39.9	65.5
12 Months or Greater Fair Value	590.1	684.9
12 Months or Greater Gross Unrealized Loss	66.7	110.8
Redeemable Preferred Stocks
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months Fair Value	0	0
Less than 12 Months Gross Unrealized Loss	0	0
12 Months or Greater Fair Value	6.9	20.9
12 Months or Greater Gross Unrealized Loss	$ 0.6	$ 1.9