Reinsurance Ceded - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011
Reinsurance [Line Items]
Loss and loss adjustment expenses	$ 157.2	$ 84.4
RSUI Group, Inc
Reinsurance [Line Items]
Description of reinsurance program	The new reinsurance program provides coverage in three layers for $500.0 million of losses in excess of a $100.0 million net retention after application of the surplus share treaties, facultative reinsurance and per risk covers. The first layer provides coverage for $100.0 million of losses, before a 60.0 percent co-participation by RSUI (compared with 47.0 percent co-participation under the expired program), in excess of the $100.0 million net retention, and the second layer provides coverage for $300.0 million of losses, before a 5.0 percent co-participation by RSUI (the same percent co-participation under the expired program), in excess of $200.0 million. RSUI expanded the new catastrophe reinsurance program to provide a new third layer of coverage of $100 million in excess of $500 million, with no co-participation by RSUI. In addition, RSUI’s property per risk reinsurance program for the 2012-2013 period provides RSUI with coverage for $90.0 million of losses, before a 10.0 percent co-participation by RSUI, in excess of a $10.0 million net retention per risk after application of the surplus share treaties and facultative reinsurance.
RSUI Group, Inc | Current Catastrophe Reinsurance Program
Reinsurance [Line Items]
Reinsurance coverage before co-participation	500.0
Net retention	100.0
RSUI Group, Inc | Property Per Risk
Reinsurance [Line Items]
Reinsurance coverage before co-participation	90.0
Net retention	10.0
Percentage of co-participation	10.00%
RSUI Group, Inc | Layer 1 | Current Catastrophe Reinsurance Program
Reinsurance [Line Items]
Reinsurance coverage before co-participation	100.0
Net retention	100.0
Percentage of co-participation	60.00%
RSUI Group, Inc | Layer 1 | Expired Catastrophe Reinsurance Program
Reinsurance [Line Items]
Percentage of co-participation	47.00%
RSUI Group, Inc | Layer 2 | Current Catastrophe Reinsurance Program
Reinsurance [Line Items]
Reinsurance coverage before co-participation	300.0
Net retention	200.0
Percentage of co-participation	5.00%
RSUI Group, Inc | Layer 2 | Expired Catastrophe Reinsurance Program
Reinsurance [Line Items]
Percentage of co-participation	5.00%
RSUI Group, Inc | Layer 3 | Current Catastrophe Reinsurance Program
Reinsurance [Line Items]
Reinsurance coverage before co-participation	100.0
Net retention	$ 500.0