Reinsurance Ceded - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013 Entity	Jun. 30, 2012	Jun. 30, 2013 Entity	Jun. 30, 2012
Effects of Reinsurance [Line Items]
Ceded premiums earned	$ 115.3	$ 120.7	$ 226.3	$ 185.8
Ceded losses incurred	$ 77.6	$ 61.7	$ 131.1	$ 123.2
Percentage of reinsurance recoverable of 20 largest reinsurers			73.40%
Number of largest reinsurers	20		20