Length of Time Fixed Maturity Securities had been in a Gross Unrealized Loss Position (Detail) (USD $) In Millions	Jun. 30, 2011	Dec. 31, 2010
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months Fair Value	$ 2,492.7	$ 3,467.6
Less than 12 Months Gross Unrealized Loss	83.4	121.3
12 Months or Greater Fair Value	929.3	1,290.1
12 Months or Greater Gross Unrealized Loss	88.8	116.1
United States Government and Government Agencies and Authorities
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months Fair Value	64.1	23.9
Less than 12 Months Gross Unrealized Loss	5.0	3.1
12 Months or Greater Fair Value	10.9	10.9
12 Months or Greater Gross Unrealized Loss	5.7	5.5
States, Municipalities, and Political Subdivisions
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months Fair Value	306.9	660.6
Less than 12 Months Gross Unrealized Loss	12.3	28.4
12 Months or Greater Fair Value	105.4	100.3
12 Months or Greater Gross Unrealized Loss	13.8	18.9
Foreign Governments
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months Fair Value	7.6
Less than 12 Months Gross Unrealized Loss	0.1
12 Months or Greater Fair Value	0
12 Months or Greater Gross Unrealized Loss	0
Public Utilities
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months Fair Value	681.9	1,073.8
Less than 12 Months Gross Unrealized Loss	29.3	41.0
12 Months or Greater Fair Value	30.8	41.0
12 Months or Greater Gross Unrealized Loss	1.5	3.3
Mortgage/Asset-backed Securities
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months Fair Value	63.9	34.5
Less than 12 Months Gross Unrealized Loss	1.6	0.1
12 Months or Greater Fair Value	25.4	45.5
12 Months or Greater Gross Unrealized Loss	0.9	0.5
All Other Corporate Bonds
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months Fair Value	1,360.5	1,667.2
Less than 12 Months Gross Unrealized Loss	34.9	48.3
12 Months or Greater Fair Value	734.9	1,071.7
12 Months or Greater Gross Unrealized Loss	66.0	85.8
Redeemable Preferred Stocks
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months Fair Value	7.8	7.6
Less than 12 Months Gross Unrealized Loss	0.2	0.4
12 Months or Greater Fair Value	21.9	20.7
12 Months or Greater Gross Unrealized Loss	$ 0.9	$ 2.1