Length of Time Fixed Maturity Securities had been in a Gross Unrealized Loss Position (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months Fair Value	$ 2,041.7	$ 3,467.6
Less than 12 Months Gross Unrealized Loss	79.5	121.3
12 Months or Greater Fair Value	949.2	1,290.1
12 Months or Greater Gross Unrealized Loss	142.7	116.1
United States Government and Government Agencies and Authorities
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months Fair Value	0	23.9
Less than 12 Months Gross Unrealized Loss	0	3.1
12 Months or Greater Fair Value	6.3	10.9
12 Months or Greater Gross Unrealized Loss	1.1	5.5
States, Municipalities, and Political Subdivisions
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months Fair Value	51.6	660.6
Less than 12 Months Gross Unrealized Loss	1.3	28.4
12 Months or Greater Fair Value	75.3	100.3
12 Months or Greater Gross Unrealized Loss	7.5	18.9
Public Utilities
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months Fair Value	192.0	1,073.8
Less than 12 Months Gross Unrealized Loss	7.9	41.0
12 Months or Greater Fair Value	142.2	41.0
12 Months or Greater Gross Unrealized Loss	20.7	3.3
Mortgage/Asset-backed Securities
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months Fair Value	94.2	34.5
Less than 12 Months Gross Unrealized Loss	4.8	0.1
12 Months or Greater Fair Value	19.6	45.5
12 Months or Greater Gross Unrealized Loss	0.7	0.5
All Other Corporate Bonds
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months Fair Value	1,703.9	1,667.2
Less than 12 Months Gross Unrealized Loss	65.5	48.3
12 Months or Greater Fair Value	684.9	1,071.7
12 Months or Greater Gross Unrealized Loss	110.8	85.8
Redeemable Preferred Stocks
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months Fair Value	0	7.6
Less than 12 Months Gross Unrealized Loss	0	0.4
12 Months or Greater Fair Value	20.9	20.7
12 Months or Greater Gross Unrealized Loss	$ 1.9	$ 2.1