Segment And Line of Business Breakdowns of Prior Period Net Reserve Deficiency (Redundancy) (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustment of Opening Balance [Line Items]
Net reserve deficiencies (redundancies) in prior years	$ (45,291)	$ 2,145	$ (13,802)
Insurance Companies
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustment of Opening Balance [Line Items]
Net reserve deficiencies (redundancies) in prior years	1,896	12,102	(5,455)
Insurance Companies | Marine
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustment of Opening Balance [Line Items]
Net reserve deficiencies (redundancies) in prior years	(10,010)	1,348	(4,155)
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	4,293	(6,828)	(14,923)
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	7,613	17,582	13,623
Lloyd's Operations
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustment of Opening Balance [Line Items]
Net reserve deficiencies (redundancies) in prior years	(47,187)	(9,957)	(8,347)
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	(30,735)	(10,311)	(3,152)
Lloyd's Operations | Property Casualty
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustment of Opening Balance [Line Items]
Net reserve deficiencies (redundancies) in prior years	(6,890)	(5,434)	(5,236)
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	$ (9,562)	$ 5,788	$ 41