Reinsurance Ceded - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012 Entity	Jun. 30, 2011	Jun. 30, 2012 Entity	Jun. 30, 2011
Reinsurance Recoverable for Insurance Companies by Product Segment [Line Items]
Ceded premiums earned	$ 120.7	$ 82.1	$ 185.8	$ 178.1
Ceded losses incurred	$ 61.7	$ 53.1	$ 123.2	$ 127.7
Percentage of reinsurance recoverable of 20 largest reinsurers			73.40%
Number of largest reinsurers	20		20