Gross Unrealized Investment Losses and Related Fair Value for Debt Securities and Equity Securities (Detail) (USD $) In Millions	Jun. 30, 2011	Dec. 31, 2010
Schedule of Available-for-sale Securities [Line Items]
Debt and Equity securities in unrealized loss position, fair value	$ 733.7	$ 864.6
Debt and Equity securities in unrealized loss position, gross losses	25.7	30.4
Debt Securities
Schedule of Available-for-sale Securities [Line Items]
Debt and Equity securities in unrealized loss position for less than 12 months, fair value	406.7	677.9
Debt and Equity securities in unrealized loss position for more than 12 months, fair value	39.1	47.2
Debt and Equity securities in unrealized loss position for less than 12 months, gross losses	5.7	20.7
Debt and Equity securities in unrealized loss position for more than 12 months, gross losses	3.5	4.1
Debt Securities | Mortgage and Asset Backed Securities
Schedule of Available-for-sale Securities [Line Items]
Debt and Equity securities in unrealized loss position for less than 12 months, fair value	102.6	170.8
Debt and Equity securities in unrealized loss position for more than 12 months, fair value	32.4	39.5
Debt and Equity securities in unrealized loss position for less than 12 months, gross losses	0.9	2.8
Debt and Equity securities in unrealized loss position for more than 12 months, gross losses	3.0	3.5
Debt Securities | U.S. Government obligations
Schedule of Available-for-sale Securities [Line Items]
Debt and Equity securities in unrealized loss position for less than 12 months, fair value	34.7	49.7
Debt and Equity securities in unrealized loss position for less than 12 months, gross losses	0.4	1.2
Debt Securities | States, municipalities and political subdivision bonds
Schedule of Available-for-sale Securities [Line Items]
Debt and Equity securities in unrealized loss position for less than 12 months, fair value	197.2	349.1
Debt and Equity securities in unrealized loss position for more than 12 months, fair value	6.7	7.7
Debt and Equity securities in unrealized loss position for less than 12 months, gross losses	2.6	14.4
Debt and Equity securities in unrealized loss position for more than 12 months, gross losses	0.5	0.6
Debt Securities | Foreign bonds
Schedule of Available-for-sale Securities [Line Items]
Debt and Equity securities in unrealized loss position for less than 12 months, fair value	15.7	45.2
Debt and Equity securities in unrealized loss position for less than 12 months, gross losses	0.7	0.9
Debt Securities | Corporate bonds and other
Schedule of Available-for-sale Securities [Line Items]
Debt and Equity securities in unrealized loss position for less than 12 months, fair value	56.5	63.1
Debt and Equity securities in unrealized loss position for less than 12 months, gross losses	1.1	1.4
Equity Securities | Common Stock
Schedule of Available-for-sale Securities [Line Items]
Debt and Equity securities in unrealized loss position for less than 12 months, fair value	287.9	139.5
Debt and Equity securities in unrealized loss position for less than 12 months, gross losses	16.5	5.6
Impaired Securities
Schedule of Available-for-sale Securities [Line Items]
Debt and Equity securities in unrealized loss position for less than 12 months, fair value	694.6	817.4
Debt and Equity securities in unrealized loss position for more than 12 months, fair value	39.1	47.2
Debt and Equity securities in unrealized loss position for less than 12 months, gross losses	22.2	26.3
Debt and Equity securities in unrealized loss position for more than 12 months, gross losses	$ 3.5	$ 4.1