Length of Time Fixed Maturity Securities had been in a Gross Unrealized Loss Position (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months Fair Value	$ 403.2	$ 2,041.7
Less than 12 Months Gross Unrealized Loss	19.3	79.5
12 Months or Greater Fair Value	788.8	949.2
12 Months or Greater Gross Unrealized Loss	71.7	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	6.6	6.3
12 Months or Greater Gross Unrealized Loss	0.9	1.1
States, Municipalities, and Political Subdivisions
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months Fair Value	1.6	51.6
Less than 12 Months Gross Unrealized Loss	0	1.3
12 Months or Greater Fair Value	42.3	75.3
12 Months or Greater Gross Unrealized Loss	7.1	7.5
Public Utilities
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months Fair Value	109.3	192.0
Less than 12 Months Gross Unrealized Loss	7.5	7.9
12 Months or Greater Fair Value	160.2	142.2
12 Months or Greater Gross Unrealized Loss	14.8	20.7
Mortgage/Asset-backed Securities
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months Fair Value	0	94.2
Less than 12 Months Gross Unrealized Loss	0	4.8
12 Months or Greater Fair Value	8.3	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	292.3	1,703.9
Less than 12 Months Gross Unrealized Loss	11.8	65.5
12 Months or Greater Fair Value	571.4	684.9
12 Months or Greater Gross Unrealized Loss	48.4	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