Net Prior Period Reserve Redundancies (Detail) (USD $) In Thousands, unless otherwise specified	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	$ 45,291	$ (2,145)
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	47,187	9,957
Lloyd's | 2010
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustment of Opening Balance [Line Items]
Net reserve deficiencies (redundancies) in prior years		5,027
Lloyd's | 2009
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustment of Opening Balance [Line Items]
Net reserve deficiencies (redundancies) in prior years		25,034
Lloyd's | 2008 and Prior
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustment of Opening Balance [Line Items]
Net reserve deficiencies (redundancies) in prior years		17,126
Marine | Lloyd's
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustment of Opening Balance [Line Items]
Net reserve deficiencies (redundancies) in prior years	2,998	30,735	10,311
Marine | Lloyd's | 2010
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustment of Opening Balance [Line Items]
Net reserve deficiencies (redundancies) in prior years		3,492
Marine | Lloyd's | 2009
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustment of Opening Balance [Line Items]
Net reserve deficiencies (redundancies) in prior years		14,792
Marine | Lloyd's | 2008 and Prior
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustment of Opening Balance [Line Items]
Net reserve deficiencies (redundancies) in prior years		12,451
Property And Casualty | Lloyd's
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustment of Opening Balance [Line Items]
Net reserve deficiencies (redundancies) in prior years		6,890
Property And Casualty | Lloyd's | 2010
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustment of Opening Balance [Line Items]
Net reserve deficiencies (redundancies) in prior years		378
Property And Casualty | Lloyd's | 2009
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustment of Opening Balance [Line Items]
Net reserve deficiencies (redundancies) in prior years		4,170
Property And Casualty | Lloyd's | 2008 and Prior
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustment of Opening Balance [Line Items]
Net reserve deficiencies (redundancies) in prior years		2,342
Professional Liability | Lloyd's
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustment of Opening Balance [Line Items]
Net reserve deficiencies (redundancies) in prior years		9,562
Professional Liability | Lloyd's | 2010
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustment of Opening Balance [Line Items]
Net reserve deficiencies (redundancies) in prior years		1,157
Professional Liability | Lloyd's | 2009
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustment of Opening Balance [Line Items]
Net reserve deficiencies (redundancies) in prior years		6,072
Professional Liability | Lloyd's | 2008 and Prior
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustment of Opening Balance [Line Items]
Net reserve deficiencies (redundancies) in prior years		$ 2,333