Reinsurance - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2012
Property Catastrophe Excess of Loss [Member]
Reinsurance [Line Items]
Effective Treaty Date	Jun. 01, 2012
Reinsurance coverage for excess of risk retention per incident, effective treaty	$ 80.0
Reinsurance coverage for risk retention per incident, effective treaty	20.0
Reinsurance percentage of one full reinstatement, effective treaty	100.00%
Reinsurance coverage for excess of risk retention per incident, terminated treaty	80.0
Reinsurance coverage for risk retention per incident, terminated treaty	20.0
Terminated Treaty Date	May 31, 2012
Reinsurance percentage occurrence coverage, terminated treaty	100.00%
Property Catastrophe Excess of Loss [Member] | Reinsurance Program Layer One [Member]
Reinsurance [Line Items]
Reinsurance coverage for excess of risk retention per incident, effective treaty	20.0
Reinsurance percentage retained, effective treaty	50.00%
Reinsurance coverage for risk retention per incident, effective treaty, layer 1	20.0
Property Catastrophe Excess of Loss [Member] | Reinsurance Program Layer Two [Member]
Reinsurance [Line Items]
Reinsurance coverage for excess of risk retention per incident, effective treaty	50.0
Reinsurance coverage for risk retention per incident, effective treaty	40.0
Reinsurance percentage retained, effective treaty	20.00%
Casualty and Professional Liability Excess Of Loss [Member]
Reinsurance [Line Items]
Effective Treaty Date	May 01, 2012
Terminated Treaty Date	Apr. 30, 2012
Reinsurance coverage for excess of risk retention per incident, effective treaty casualty section	2.0
Reinsurance coverage for risk retention per incident, effective treaty casualty section	1.0
Reinsurance coverage for excess of risk retention per incident, effective treaty professional liability section	4.0
Reinsurance coverage for risk retention per incident, effective treaty professional liability section	1.0
Property Per Risk Excess Of Loss (Member)
Reinsurance [Line Items]
Effective Treaty Date	Jan. 01, 2012
Reinsurance coverage for risk retention per incident, effective treaty, layer 1	2.0
Terminated Treaty Date	Dec. 31, 2011
Reinsurance coverage for excess of risk retention per incident, effective treaty	13.0
Reinsurance coverage for risk retention per incident, effective treaty	2.0
Reinsurance coverage for excess of risk retention per incident, effective treaty, layer 1	3.0
Reinsurance coverage for excess of risk retention per incident, effective treaty, layer 2	10.0
Reinsurance coverage for risk retention per incident, effective treaty, layer 2	5.0
Limit of liability for risks involved in one loss occurrence	3.0
Limit of liability for risks involved in one loss occurrence	10.0
Professional Liability Excess Of Loss [Member]
Reinsurance [Line Items]
Effective Treaty Date	May 01, 2011
Reinsurance coverage for excess of risk retention per incident, terminated treaty	4.0
Reinsurance coverage for risk retention per incident, terminated treaty	1.0
Terminated Treaty Date	Apr. 30, 2011
Casualty Clash Excess of Loss (Member)
Reinsurance [Line Items]
Effective Treaty Date	May 01, 2012
Reinsurance coverage for excess of risk retention per incident, effective treaty	10.0
Reinsurance coverage for risk retention per incident, effective treaty	3.0
Terminated Treaty Date	Apr. 30, 2012
Limit of liability for risks involved in one loss occurrence	20.0
Marine Excess Of Loss [Member]
Reinsurance [Line Items]
Effective Treaty Date	May 24, 2010
Reinsurance coverage for risk retention per incident, effective treaty, layer 1	2.0
Terminated Treaty Date	Nov. 30, 2011
Reinsurance coverage for excess of risk retention per incident, effective treaty	13.0
Reinsurance coverage for risk retention per incident, effective treaty	2.0
Reinsurance coverage for excess of risk retention per incident, effective treaty, layer 1	3.0
Reinsurance coverage for excess of risk retention per incident, effective treaty, layer 2	5.0
Reinsurance coverage for risk retention per incident, effective treaty, layer 2	5.0
Reinsurance percentage of two full reinstatement, effective treaty, Layer 1 and 2	100.00%
Reinsurance coverage for excess of risk retention per incident, effective treaty, layer 3	5.0
Reinsurance coverage for risk retention per incident, effective treaty, layer 3	$ 10.0
Reinsurance percentage of one full reinstatement, Effective Treaty, Layer 3	100.00%