Reinsurance Ceded - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013 Entity	Sep. 30, 2012	Sep. 30, 2013 Entity	Sep. 30, 2012
Effects of Reinsurance [Line Items]
Ceded premiums earned	$ 114.2	$ 97.4	$ 340.5	$ 283.4
Ceded losses incurred	$ (37.0)	$ 31.2	$ 94.1	$ 154.4
Percentage of reinsurance recoverable of 20 largest reinsurers			76.30%
Number of largest reinsurers	20		20