Effect of Reinsurance on Premiums Written and Earned (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Direct premiums written	$ 16,558.8	$ 15,333.1	$ 14,700.0
Ceded written	(186.1)	(186.5)	(223.2)
Net premiums written	16,372.7	15,146.6	14,476.8
Direct premiums earned	16,207.6	15,107.5	14,519.2
Ceded earned	(189.6)	(204.7)	(204.4)
Net premiums earned	$ 16,018.0	$ 14,902.8	$ 14,314.8

Premiums assumed and earned by another insurer as a result of reinsurance arrangements.

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Total consideration paid and payable to reinsurers during the period pursuant to terms of reinsurance arrangements.

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Premiums earned directly for all property and casualty insurance before adding contracts assumed from other insurers or subtracting any amounts assumed by other insurers.

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Total consideration paid and payable by direct policyholders for insurance contracts sold by the Entity during the period. This amount does not include premium assumed from or ceded to other insurers.

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