Property and Casualty Premiums Written and Earned (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Premiums written
Premiums direct	$ 1,088.6	$ 1,080.5	$ 1,238.8
Premiums assumed	52.4	23.5	20.3
Premiums ceded	366.3	367.8	428.3
Premiums earned
Premiums direct	1,069.1	1,131.7	1,278.9
Premiums assumed	45.9	21.0	19.1
Premiums ceded	$ 367.4	$ 384.6	$ 453.0

Premiums assumed and earned for all property and casualty insurance assumed from other insurers as a result of reinsurance arrangements.

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