Reserves for Losses and Loss Adjustment Expenses - Additional Information (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 | Marine | Marine Liability Business
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustment of Opening Balance [Line Items]
Net reserve deficiencies (redundancies) in prior years		(2,600)
Insurance Companies | Marine | Hull
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustment of Opening Balance [Line Items]
Net reserve deficiencies (redundancies) in prior years		(1,400)
Insurance Companies | Inland Marine Systems
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustment of Opening Balance [Line Items]
Net reserve deficiencies (redundancies) in prior years	4,000
Insurance Companies | Ocean Marine Systems
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustment of Opening Balance [Line Items]
Net reserve deficiencies (redundancies) in prior years	(2,700)
Insurance Companies | Ocean Marine Systems | Craft, P&I, and Transport classes
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustment of Opening Balance [Line Items]
Net reserve deficiencies (redundancies) in prior years	(5,800)
Insurance Companies | Ocean Marine Systems | Specie and Liability classes
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustment of Opening Balance [Line Items]
Net reserve deficiencies (redundancies) in prior years	3,200
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 | Property Casualty | Offshore Energy
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustment of Opening Balance [Line Items]
Net reserve deficiencies (redundancies) in prior years	8,400
Insurance Companies | Property Casualty | NavTech
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustment of Opening Balance [Line Items]
Net reserve deficiencies (redundancies) in prior years		(2,900)
Insurance Companies | Property Casualty | West Coast Contractors' Liability
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustment of Opening Balance [Line Items]
Net reserve deficiencies (redundancies) in prior years		(29,200)
Insurance Companies | Property Casualty | Somerset Re Run-Off Book
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustment of Opening Balance [Line Items]
Net reserve deficiencies (redundancies) in prior years		(1,800)
Insurance Companies | Property Casualty | Agriculture Reinsurance Book
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustment of Opening Balance [Line Items]
Net reserve deficiencies (redundancies) in prior years		(1,500)
Insurance Companies | Property Casualty | Specialty Run-Off Books
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustment of Opening Balance [Line Items]
Net reserve deficiencies (redundancies) in prior years		16,500
Insurance Companies | Property Casualty | Personal Umbrella Lines
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustment of Opening Balance [Line Items]
Net reserve deficiencies (redundancies) in prior years		13,300
Insurance Companies | Property Casualty | Liquor Business
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustment of Opening Balance [Line Items]
Net reserve deficiencies (redundancies) in prior years		2,000
Insurance Companies | Property Casualty | New York Construction Liability
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustment of Opening Balance [Line Items]
Net reserve deficiencies (redundancies) in prior years		1,700
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
Insurance Companies | Professional Liability | Management Liability
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustment of Opening Balance [Line Items]
Net reserve deficiencies (redundancies) in prior years	14,500
Insurance Companies | Professional Liability | Errors and Omissions
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustment of Opening Balance [Line Items]
Net reserve deficiencies (redundancies) in prior years	3,100
Insurance Companies | Professional Liability | Directors and Officers Liability Book of Business
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustment of Opening Balance [Line Items]
Net reserve deficiencies (redundancies) in prior years		15,700
Insurance Companies | Professional Liability | Run Off Lawyers Book of Business
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustment of Opening Balance [Line Items]
Net reserve deficiencies (redundancies) in prior years		(1,400)
Insurance Companies | Professional Liability | Other Lawyers Business
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustment of Opening Balance [Line Items]
Net reserve deficiencies (redundancies) in prior years		(700)
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 | NavTech
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustment of Opening Balance [Line Items]
Net reserve deficiencies (redundancies) in prior years	(5,500)	(4,800)
Lloyd's Operations | All Other
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustment of Opening Balance [Line Items]
Net reserve deficiencies (redundancies) in prior years		(500)
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