Significant Accounting Policies (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Reclassification out of Accumulated Other Comprehensive Income [Member] \| Unrealized gains (losses) on investments
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Reclassification adjustment for investments transferred to equity method	$ 4,531	$ 0	$ 4,531	$ 0

Amount of reclassification adjustment from accumulated other comprehensive income for unrealized gain (loss) of investments classified as available-for-sale transferred to equity method.

No definition available.

Name:	xlgroup_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAociForInvestmentsTransferredFromAvailableforsaleToEquityMethod
Namespace Prefix:	xlgroup_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration