Segment And Line of Business Breakdowns of Prior Period Net Reserve Deficiency (Redundancy) (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Liability for Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years	$ 55,812	$ 1,266	$ 45,291
Insurance Companies
Liability for Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years	(30,312)	13,430	1,896
Insurance Companies | Marine
Liability for Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years	(41,388)	(15,227)	(10,010)
Insurance Companies | Property Casualty
Liability for Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years	14,612	18,466	4,293
Insurance Companies | Professional Liability
Liability for Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years	(3,536)	10,191	7,613
Lloyd's
Liability for Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years	(25,500)	(14,696)	(47,187)
Lloyd's | Marine
Liability for Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years	(21,336)	(2,998)	(30,735)
Lloyd's | Property Casualty
Liability for Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years	(1,500)	(14,574)	(6,890)
Lloyd's | Professional Liability
Liability for Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years	$ (2,664)	$ 2,876	$ (9,562)