Significant Accounting Policies (Details) (Accounting Standards Update 2010-26 [Member], USD $) In Millions, unless otherwise specified	9 Months Ended
Sep. 30, 2012
Accounting Standards Update 2010-26 [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Deferred tax liability, cumulative adjustment effect	$ 7.0
Deferred acquisition costs, cumulative adjustment effect	21.0
Net assets, cumulative adjustment effect	$ 14.0

Amount of the cumulative effect of the change in accounting principle or new accounting pronouncement on retained earnings or other components of equity or net assets in the statement of financial position as of the beginning of the earliest period presented.

No definition available.

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