Length of Time Fixed Maturity Securities had been in a Gross Unrealized Loss Position (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months Fair Value	$ 586.8	$ 2,041.7
Less than 12 Months Gross Unrealized Loss	11.8	79.5
12 Months or Greater Fair Value	596.8	949.2
12 Months or Greater Gross Unrealized Loss	44.2	142.7
United States Government and Government Agencies and Authorities
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months Fair Value	0	0
Less than 12 Months Gross Unrealized Loss	0	0
12 Months or Greater Fair Value	6.5	6.3
12 Months or Greater Gross Unrealized Loss	1.1	1.1
States, Municipalities, and Political Subdivisions
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months Fair Value	30.8	51.6
Less than 12 Months Gross Unrealized Loss	0.9	1.3
12 Months or Greater Fair Value	42.1	75.3
12 Months or Greater Gross Unrealized Loss	6.9	7.5
Public Utility, Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months Fair Value	110.3	192.0
Less than 12 Months Gross Unrealized Loss	3.9	7.9
12 Months or Greater Fair Value	147.6	142.2
12 Months or Greater Gross Unrealized Loss	11.2	20.7
Mortgage/Asset-backed Securities
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months Fair Value	4.4	94.2
Less than 12 Months Gross Unrealized Loss	0	4.8
12 Months or Greater Fair Value	3.8	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	441.3	1,703.9
Less than 12 Months Gross Unrealized Loss	7.0	65.5
12 Months or Greater Fair Value	396.8	684.9
12 Months or Greater Gross Unrealized Loss	24.5	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