Segment Break Down of Prior Years' Net Reserve Deficiency (Redundancy) (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustment of Opening Balance [Line Items]
Net reserve deficiencies (redundancies) in prior years	$ 2,145	$ (13,802)	$ (8,941)
Insurance Companies
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustment of Opening Balance [Line Items]
Net reserve deficiencies (redundancies) in prior years	12,102	(5,455)	(3,079)
Insurance Companies | Marine
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustment of Opening Balance [Line Items]
Net reserve deficiencies (redundancies) in prior years	1,348	(4,155)	11,893
Insurance Companies | Property Casualty
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustment of Opening Balance [Line Items]
Net reserve deficiencies (redundancies) in prior years	(6,828)	(14,923)	(35,658)
Insurance Companies | Professional Liability
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustment of Opening Balance [Line Items]
Net reserve deficiencies (redundancies) in prior years	17,582	13,623	20,686
Lloyd's Operations
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustment of Opening Balance [Line Items]
Net reserve deficiencies (redundancies) in prior years	(9,957)	(8,347)	(5,862)
Lloyd's Operations | Marine
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustment of Opening Balance [Line Items]
Net reserve deficiencies (redundancies) in prior years	(10,300)	(3,200)
Lloyd's Operations | Professional Liability
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustment of Opening Balance [Line Items]
Net reserve deficiencies (redundancies) in prior years	$ 5,800