Reinsurance Ceded - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Ceded Credit Risk [Line Items]
Loss and loss adjustment expenses	$ 38.0	$ 23.6
Description of reinsurance program	The new reinsurance program provides coverage in two layers for $400.0 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. 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
Ceded Credit Risk [Line Items]
Reinsurance coverage capacity	90.0
Amount of catastrophe losses, net retention per risk	10.0
Percentage of co-participation	10.00%
New reinsurance program
Ceded Credit Risk [Line Items]
Reinsurance coverage capacity	400.0
Amount of catastrophe losses, net retention per risk	100.0
New reinsurance program | Reinsurance Program Layer One
Ceded Credit Risk [Line Items]
Reinsurance coverage capacity	100.0
Percentage of co-participation	60.00%
Expired reinsurance program | Reinsurance Program Layer One
Ceded Credit Risk [Line Items]
Percentage of co-participation	47.00%
Expired reinsurance program | Reinsurance Program Layer Two
Ceded Credit Risk [Line Items]
Percentage of co-participation	5.00%
New reinsurance program | Minimum
Ceded Credit Risk [Line Items]
Reinsurance coverage capacity	200.0
New reinsurance program | Reinsurance Program Layer Two
Ceded Credit Risk [Line Items]
Reinsurance coverage capacity	$ 300.0
Percentage of co-participation	5.00%