Length of Time Fixed Maturity Securities had been in a Gross Unrealized Loss Position (Detail) (USD $) In Millions	Sep. 30, 2011	Dec. 31, 2010
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months Fair Value	$ 2,627.7	$ 3,467.6
Less than 12 Months Gross Unrealized Loss	141.7	121.3
12 Months or Greater Fair Value	852.1	1,290.1
12 Months or Greater Gross Unrealized Loss	127.9	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	6.2	10.9
12 Months or Greater Gross Unrealized Loss	1.2	5.5
States, Municipalities, and Political Subdivisions
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months Fair Value	4.8	660.6
Less than 12 Months Gross Unrealized Loss	0.1	28.4
12 Months or Greater Fair Value	89.8	100.3
12 Months or Greater Gross Unrealized Loss	3.0	18.9
Public Utilities
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months Fair Value	211.8	1,073.8
Less than 12 Months Gross Unrealized Loss	19.7	41.0
12 Months or Greater Fair Value	74.7	41.0
12 Months or Greater Gross Unrealized Loss	8.9	3.3
Mortgage/Asset-backed Securities
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months Fair Value	95.5	34.5
Less than 12 Months Gross Unrealized Loss	3.1	0.1
12 Months or Greater Fair Value	20.8	45.5
12 Months or Greater Gross Unrealized Loss	0.8	0.5
All Other Corporate Bonds
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months Fair Value	2,315.6	1,667.2
Less than 12 Months Gross Unrealized Loss	118.8	48.3
12 Months or Greater Fair Value	639.5	1,071.7
12 Months or Greater Gross Unrealized Loss	112.3	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	21.1	20.7
12 Months or Greater Gross Unrealized Loss	$ 1.7	$ 2.1

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This item represents the excess of amortized cost basis over fair value of securities that have been in a loss position for less than twelve months for those securities which are categorized neither as held-to-maturity nor trading securities.

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This item represents the aggregate fair value of investments in debt and equity securities categorized neither as held-to-maturity nor trading securities that have been in a continuous unrealized loss position for less than twelve months.

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