Reinsurance Ceded - Additional Information (Detail) (RSUI, USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013
Reinsurance [Line Items]
Description of reinsurance program	The new catastrophe 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, in excess of the $100.0 million net retention, the second layer provides coverage for $300.0 million of losses, before a 5.0 percent co-participation by RSUI, in excess of $200.0 million and the third layer provides coverage for $100.0 million of losses in excess of $500.0 million, with no co-participation by RSUI. In addition, RSUI's property per risk reinsurance program for the 2013-2014 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
Current Catastrophe Reinsurance Program
Reinsurance [Line Items]
Reinsurance coverage before co-participation	500.0
Net retention	100.0
Current Catastrophe Reinsurance Program | Layer 1
Reinsurance [Line Items]
Reinsurance coverage before co-participation	100.0
Net retention	100.0
Percentage of co-participation	60.00%
Current Catastrophe Reinsurance Program | Layer 2
Reinsurance [Line Items]
Reinsurance coverage before co-participation	300.0
Net retention	200.0
Percentage of co-participation	5.00%
Current Catastrophe Reinsurance Program | Layer 3
Reinsurance [Line Items]
Reinsurance coverage before co-participation	100.0
Net retention	500.0
Property Per Risk
Reinsurance [Line Items]
Reinsurance coverage before co-participation	90.0
Net retention	10.0
Percentage of co-participation	10.00%