Reserve for losses and loss expenses (Narrative) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2016	Dec. 31, 2013
Liability for Claims and Claims Adjustment Expense [Line Items]
Reserve for losses and loss expenses	$ 2,996,567	$ 3,243,147	$ 2,995,195	$ 3,047,933
Western World Insurance Group, Inc
Liability for Claims and Claims Adjustment Expense [Line Items]
Fair value adjustment to net reserves	15,586
Amortized to income during the period	$ 10,979	$ 4,607
AlphaCat
Liability for Claims and Claims Adjustment Expense [Line Items]
Percentage of Total Reserves			1.90%
Reserve for losses and loss expenses			$ 48,534
Property | Western World Insurance Group, Inc
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for losses and loss expenses, before 2012, net of reinsurance			0
Property | Validus Re
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for losses and loss expenses, before 2012, net of reinsurance			154,539
Property | Flagstone [Member] | Validus Re
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for losses and loss expenses, before 2012, net of reinsurance			77,790
Property | IPC [Member] | Validus Re
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for losses and loss expenses, before 2012, net of reinsurance			10,614
Marine | Validus Re
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for losses and loss expenses, before 2012, net of reinsurance			64,750
Marine | Flagstone [Member] | Validus Re
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for losses and loss expenses, before 2012, net of reinsurance			15,178
Marine | IPC [Member] | Validus Re
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for losses and loss expenses, before 2012, net of reinsurance			1,537
Specialty | Validus Re
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for losses and loss expenses, before 2012, net of reinsurance			67,909
Specialty | Flagstone [Member] | Validus Re
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for losses and loss expenses, before 2012, net of reinsurance			41,091
Specialty | IPC [Member] | Validus Re
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for losses and loss expenses, before 2012, net of reinsurance			$ 5,743