Significant Accounting Policies (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
Sep. 30, 2014
Adjustment to premiums earned and associated tax accruals, amount	$ 20

The amount of adjustment, net of tax, to premiums earned and associated tax accruals related to reinstatement premiums due under assumed reinsurance contracts arising from unpaid losses and loss expenses reported in a prior period.

No definition available.

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