Reserves for Losses and Loss Adjustment Expenses - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustment of Opening Balance [Line Items]
Net reserve deficiencies (redundancies) in prior years	$ (1,266,000)	$ (45,291,000)	$ 2,145,000
Marine | 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
Marine | 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 | 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
Insurance
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustment of Opening Balance [Line Items]
Net reserve deficiencies (redundancies) in prior years	13,430,000	1,896,000	12,102,000
Insurance | Offshore Energy
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustment of Opening Balance [Line Items]
Net reserve deficiencies (redundancies) in prior years	8,000,000
Insurance | Marine
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustment of Opening Balance [Line Items]
Net reserve deficiencies (redundancies) in prior years	(15,227,000)	(10,010,000)	1,348,000
Insurance | Professional Liability
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustment of Opening Balance [Line Items]
Net reserve deficiencies (redundancies) in prior years	10,191,000	7,613,000	17,582,000
Insurance | 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	6,100,000		14,500,000
Insurance | 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	4,400,000		3,100,000
Insurance | Property Casualty
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustment of Opening Balance [Line Items]
Net reserve deficiencies (redundancies) in prior years	18,466,000	4,293,000	(6,828,000)
Insurance | Property Casualty | Accident and Health
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustment of Opening Balance [Line Items]
Net reserve deficiencies (redundancies) in prior years	13,200,000	4,300,000
Insurance | 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 | Property Casualty | Small general and artisan contractors
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustment of Opening Balance [Line Items]
Net reserve deficiencies (redundancies) in prior years	10,400,000
Insurance | Property Casualty | Business in run-off
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustment of Opening Balance [Line Items]
Net reserve deficiencies (redundancies) in prior years	2,100,000
Insurance | Inland Marine
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 | Ocean Marine
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 | Ocean Marine | 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 | Ocean Marine | 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
Lloyd's
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustment of Opening Balance [Line Items]
Net reserve deficiencies (redundancies) in prior years	(14,696,000)	(47,187,000)	(9,957,000)
Impact of settlement of unpaid claims and claim adjustments		5,000,000
Lloyd's | 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)
Lloyd's | Marine
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustment of Opening Balance [Line Items]
Net reserve deficiencies (redundancies) in prior years	(2,998,000)	(30,735,000)	(10,311,000)
Impact of settlement of unpaid claims and claim adjustments		4,100,000
Lloyd's | Professional Liability
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustment of Opening Balance [Line Items]
Net reserve deficiencies (redundancies) in prior years	2,876,000	(9,562,000)	5,788,000
Impact of settlement of unpaid claims and claim adjustments		900,000
Lloyd's | 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	3,200,000
Lloyd's | 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	6,100,000
Lloyd's | Property Casualty
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustment of Opening Balance [Line Items]
Net reserve deficiencies (redundancies) in prior years	$ (14,574,000)	$ (6,890,000)	$ (5,434,000)