Reserves for Losses and Loss Adjustment Expenses - Additional Information (Detail) (USD $)	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	$ (45,291,000)	$ 2,145,000	$ (13,802,000)
Marine Business | Case Reserves
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustment of Opening Balance [Line Items]
Net reserve deficiencies (redundancies) in prior years	3,400,000
Marine Business | Claims Emergence
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustment of Opening Balance [Line Items]
Net reserve deficiencies (redundancies) in prior years	(4,000,000)
Marine Business | Short Tail Marine Lines
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustment of Opening Balance [Line Items]
Net reserve deficiencies (redundancies) in prior years	2,600,000
Insurance Companies
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustment of Opening Balance [Line Items]
Net reserve deficiencies (redundancies) in prior years	1,896,000	12,102,000	(5,455,000)
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	4,293,000	(6,828,000)	(14,923,000)
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		8,400,000
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			(2,900,000)
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			(29,200,000)
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			(1,800,000)
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			(1,500,000)
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			16,500,000
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			13,300,000
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			2,000,000
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			1,700,000
Insurance Companies | Marine
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustment of Opening Balance [Line Items]
Net reserve deficiencies (redundancies) in prior years	(10,010,000)	1,348,000	(4,155,000)
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			(2,600,000)
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			(1,400,000)
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	7,613,000	17,582,000	13,623,000
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		14,500,000
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		3,100,000
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			15,700,000
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			(1,400,000)
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			(700,000)
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		4,000,000
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		(2,700,000)
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		(5,800,000)
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		3,200,000
Insurance Companies | Marine Business
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustment of Opening Balance [Line Items]
Net reserve deficiencies (redundancies) in prior years	(10,000,000)
Insurance Companies | Accident and Health Insurance Segment
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustment of Opening Balance [Line Items]
Net reserve deficiencies (redundancies) in prior years	4,300,000
Lloyd's Operations
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustment of Opening Balance [Line Items]
Net reserve deficiencies (redundancies) in prior years	(47,187,000)	(9,957,000)	(8,347,000)
Impact of settlement of unpaid claims and claim adjustments	5,000,000
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		(5,500,000)	(4,800,000)
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			(500,000)
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	(6,890,000)	(5,434,000)	(5,236,000)
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	(30,735,000)	(10,311,000)	(3,152,000)
Impact of settlement of unpaid claims and claim adjustments	4,100,000
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	(9,562,000)	5,788,000	41,000
Impact of settlement of unpaid claims and claim adjustments	$ 900,000