Net Prior Period Reserve Redundancies (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)
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 | 2010
Liability for Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years			5,027
Lloyd's | 2009
Liability for Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years			25,034
Lloyd's | 2008 and Prior
Liability for Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years			17,126
Marine | Lloyd's
Liability for Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years	21,336	2,998	30,735
Marine | Lloyd's | 2010
Liability for Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years			3,492
Marine | Lloyd's | 2009
Liability for Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years			14,792
Marine | Lloyd's | 2008 and Prior
Liability for Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years			12,451
Property Casualty | Lloyd's
Liability for Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years	1,500	14,574	6,890
Property Casualty | Lloyd's | 2010
Liability for Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years			378
Property Casualty | Lloyd's | 2009
Liability for Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years			4,170
Property Casualty | Lloyd's | 2008 and Prior
Liability for Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years			2,342
Professional Liability | Lloyd's
Liability for Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years	2,664	(2,876)	9,562
Professional Liability | Lloyd's | 2010
Liability for Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years			1,157
Professional Liability | Lloyd's | 2009
Liability for Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years			6,072
Professional Liability | Lloyd's | 2008 and Prior
Liability for Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years			$ 2,333