Reinsurance Ceded - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012 Entity	Sep. 30, 2011	Sep. 30, 2012 Entity	Sep. 30, 2011
Reinsurance Recoverable for Insurance Companies by Product Segment [Line Items]
Ceded premiums earned	$ 97.4	$ 85.5	$ 283.4	$ 261.9
Ceded losses incurred	31.2	39.8	154.4	165.6
Adjustment to loss retention on corrections to reinsurance treaty retentions	5.3
Adjustment to reinstatement premiums related to corrections for reinsurance treaty	1.9
Pre-tax impact from corrections to reinsurance treaty retentions	3.4
After-tax impact from corrections to reinsurance treaty retentions	$ 2.2
Percentage of reinsurance recoverable of 20 largest reinsurers			73.90%
Number of largest reinsurers	20		20