Length of Time Fixed Maturity Securities had been in a Gross Unrealized Loss Position (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months Fair Value	$ 1,419.2	$ 2,041.7
Less than 12 Months Gross Unrealized Loss	45.0	79.5
12 Months or Greater Fair Value	819.1	949.2
12 Months or Greater Gross Unrealized Loss	122.8	142.7
United States Government and Government Agencies and Authorities
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	6.7	6.3
12 Months or Greater Gross Unrealized Loss	0.8	1.1
States, Municipalities, and Political Subdivisions
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months Fair Value	69.3	51.6
Less than 12 Months Gross Unrealized Loss	0.8	1.3
12 Months or Greater Fair Value	40.5	75.3
12 Months or Greater Gross Unrealized Loss	8.8	7.5
Public Utilities
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months Fair Value	251.8	192.0
Less than 12 Months Gross Unrealized Loss	13.4	7.9
12 Months or Greater Fair Value	153.9	142.2
12 Months or Greater Gross Unrealized Loss	27.5	20.7
Mortgage/Asset-backed Securities
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months Fair Value	2.5	94.2
Less than 12 Months Gross Unrealized Loss	0	4.8
12 Months or Greater Fair Value	10.8	19.6
12 Months or Greater Gross Unrealized Loss	0.5	0.7
All Other Corporate Bonds
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months Fair Value	1,095.6	1,703.9
Less than 12 Months Gross Unrealized Loss	30.8	65.5
12 Months or Greater Fair Value	607.2	684.9
12 Months or Greater Gross Unrealized Loss	85.2	110.8
Redeemable Preferred Stocks
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months Fair Value		0
Less than 12 Months Gross Unrealized Loss		0
12 Months or Greater Fair Value		20.9
12 Months or Greater Gross Unrealized Loss		$ 1.9