Net Prior Period Reserve Redundancies (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)
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 | 2010
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustment of Opening Balance [Line Items]
Net reserve deficiencies (redundancies) in prior years	5,027
Lloyd's Operations | 2009
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustment of Opening Balance [Line Items]
Net reserve deficiencies (redundancies) in prior years	25,034
Lloyd's Operations | 2008 and Prior
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustment of Opening Balance [Line Items]
Net reserve deficiencies (redundancies) in prior years	17,126
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 | Marine | 2010
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustment of Opening Balance [Line Items]
Net reserve deficiencies (redundancies) in prior years	3,492
Lloyd's Operations | Marine | 2009
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustment of Opening Balance [Line Items]
Net reserve deficiencies (redundancies) in prior years	14,792
Lloyd's Operations | Marine | 2008 and Prior
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustment of Opening Balance [Line Items]
Net reserve deficiencies (redundancies) in prior years	12,451
Lloyd's Operations | Property And Casualty
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustment of Opening Balance [Line Items]
Net reserve deficiencies (redundancies) in prior years	(6,890)
Lloyd's Operations | Property And Casualty | 2010
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustment of Opening Balance [Line Items]
Net reserve deficiencies (redundancies) in prior years	378
Lloyd's Operations | Property And Casualty | 2009
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustment of Opening Balance [Line Items]
Net reserve deficiencies (redundancies) in prior years	4,170
Lloyd's Operations | Property And Casualty | 2008 and Prior
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustment of Opening Balance [Line Items]
Net reserve deficiencies (redundancies) in prior years	2,342
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)
Lloyd's Operations | Professional Liability | 2010
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustment of Opening Balance [Line Items]
Net reserve deficiencies (redundancies) in prior years	1,157
Lloyd's Operations | Professional Liability | 2009
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustment of Opening Balance [Line Items]
Net reserve deficiencies (redundancies) in prior years	6,072
Lloyd's Operations | Professional Liability | 2008 and Prior
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustment of Opening Balance [Line Items]
Net reserve deficiencies (redundancies) in prior years	$ 2,333