Reserves for Losses and Loss Adjustment Expenses - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years	$ 55,812	$ 1,266	$ 45,291
Marine | Short Tail Marine Lines
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years			2,600
Marine | Claims Emergence
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years			4,000
Marine | Case Reserves
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years			3,400
Insurance Companies
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years	(30,312)	13,430	1,896
Insurance Companies | Offshore Energy
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years		8,000
Insurance Companies | Marine
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years	(41,388)	(15,227)	(10,010)
Insurance Companies | Marine | Marine Liability
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years	13,400
Insurance Companies | Marine | Craft/Fishing Vassels
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years	7,200
Insurance Companies | Marine | P&I
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years	6,400
Insurance Companies | Marine | Inland Marine
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years	4,700
Insurance Companies | Marine | Bluewater Hull
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years	1,100
Insurance Companies | Marine | Cargo
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years	700
Insurance Companies | Property Casualty
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years	14,612	18,466	4,293
Insurance Companies | Property Casualty | Contractors
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years	23,200	10,400
Insurance Companies | Property Casualty | Claims Emergence
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years	6,100
Insurance Companies | Property Casualty | Accident and Health
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years		13,200	4,300
Insurance Companies | Property Casualty | Business in run-off
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years		2,100
Insurance Companies | Professional Liability
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years	(3,536)	10,191	7,613
Insurance Companies | Professional Liability | Errors and Omissions
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years		4,400
Insurance Companies | Professional Liability | Management Liability
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years		6,100
Insurance Companies | Professional Liability | Settlement
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years	4,500
Lloyd's
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years	(25,500)	(14,696)	(47,187)
Lloyd's | Syndicate 1221
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years			5,000
Lloyd's | Marine
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years	(21,336)	(2,998)	(30,735)
Lloyd's | Marine | Marine Liability
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years	11,000
Lloyd's | Marine | Cargo
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years	1,000
Lloyd's | Marine | Specie
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years	3,700
Lloyd's | Marine | Transport
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years	1,300
Lloyd's | Marine | Energy Liability
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years	2,300
Lloyd's | Marine | Marine Assumed
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years	2,000
Lloyd's | Marine | Syndicate 1221
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years			4,100
Lloyd's | Property Casualty
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years	(1,500)	(14,574)	(6,890)
Lloyd's | Professional Liability
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years	(2,664)	2,876	(9,562)
Lloyd's | Professional Liability | Errors and Omissions
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years	1,000	6,100
Lloyd's | Professional Liability | Management Liability
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years	1,700	3,200
Lloyd's | Professional Liability | Syndicate 1221
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years			$ 900