Liability for Loss and Loss Adjustment Expenses - Additional Information (Detail) (USD $)	12 Months Ended													3 Months Ended		12 Months Ended												3 Months Ended	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2011 RSUI Group, Inc	Dec. 31, 2010 RSUI Group, Inc	Dec. 31, 2011 Capitol Transamerica Corporation And Platte River Insurance Company	Dec. 31, 2010 Capitol Transamerica Corporation And Platte River Insurance Company	Dec. 31, 2011 Pacific Compensation Corporation	Dec. 31, 2010 Pacific Compensation Corporation	Dec. 31, 2011 Property and Casualty Insurance	Dec. 31, 2010 Property and Casualty Insurance	Dec. 31, 2011 Workers' Compensation	Dec. 31, 2010 Workers' Compensation	Dec. 31, 2011 Workers' Compensation Pacific Compensation Corporation	Jun. 30, 2011 Workers' Compensation Pacific Compensation Corporation	Dec. 31, 2011 Workers' Compensation Pacific Compensation Corporation	Dec. 31, 2010 Workers' Compensation Pacific Compensation Corporation	Dec. 31, 2011 Casualty and Workers Compensation	Dec. 31, 2011 Casualty reserve RSUI Group, Inc	Dec. 31, 2010 Casualty reserve RSUI Group, Inc	Dec. 31, 2011 Casualty reserve Capitol Transamerica Corporation And Platte River Insurance Company	Dec. 31, 2010 Casualty reserve Capitol Transamerica Corporation And Platte River Insurance Company	Dec. 31, 2010 General Liability RSUI Group, Inc	Dec. 31, 2010 Directors and Officers Liability Insurance RSUI Group, Inc	Dec. 31, 2011 Property Risks RSUI Group, Inc	Dec. 31, 2010 Property Risks RSUI Group, Inc	Dec. 31, 2011 Property Risks Capitol Transamerica Corporation And Platte River Insurance Company	Jun. 30, 2010 Hurricane RSUI Group, Inc	Dec. 31, 2010 Hurricane RSUI Group, Inc	Dec. 31, 2010 2005 Hurricanes RSUI Group, Inc	Dec. 31, 2011 Terminated Program Business Capitol Transamerica Corporation And Platte River Insurance Company	Dec. 31, 2010 Reinsurance Transactions Capitol Transamerica Corporation And Platte River Insurance Company
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustment of Opening Balance [Line Items]
Increase (decrease) in loss and loss adjustment expenses reserve	$ 15,707,000	$ 192,237,000	$ 57,611,000							$ 42,300,000	$ 99,000,000	$ (19,200,000)	$ 59,200,000					$ (39,600,000)
Claims incurred related to prior years	(25,800,000)	(33,700,000)	(17,900,000)	(59,500,000)	(43,200,000)	5,000,000	(3,900,000)	28,700,000	13,400,000					13,400,000	15,000,000	28,400,000	12,500,000		(56,200,000)	(33,900,000)	5,000,000	(400,000)	(41,400,000)	7,500,000	(3,300,000)	(9,300,000)	(9,600,000)	5,300,000	16,300,000	11,000,000	14,600,000	(3,500,000)
The amount of lower cumulative losses, expressed as a percentage of carried loss and LAE reserves at the beginning of the year				2.30%
Decrease in ceded premiums earned								$ 1,100,000	$ 5,000,000						$ 2,000,000	$ 2,000,000