Fair Value Measurements (Detail) (USD $) In Millions	Sep. 30, 2011	Dec. 31, 2010
Fair value of floating rate medium term notes	$ 429.2	$ 454.8
Amortized cost of floating rate medium term notes	460.0	504.6
Fixed Maturities Equity Securities	91.1
Investments In Structured Transactions not Carried at Fair Value	324.7	315.4
Derivative Collateral Obligation to Return Cash	$ 54.0	$ 23.0
Discount Rate For Determining Fair Value Of Deposit Liabilities	220.0	142.3

Amortized cost value of certain floating rate medium term notes supported primarily by pools of European credit with varying degrees of leverage that are included within Corporate Fixed Maturity Securities.These notes allow the investor to participate in cash flows of the underlying bonds including certain residual values, which could serve to either decrease or increase the ultimate values of these notes.

Name:	xlgroup_AmortizedCostOfFloatingRateMediumTermNotes
Namespace Prefix:	xlgroup
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The amount of the obligation to return cash collateral under master

+ Details

Name:	xlgroup_DerivativeCollateralObligationtoReturnCash
Namespace Prefix:	xlgroup
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Discount rate used for determining fair value of deposit liabilities.

+ Details

Name:	xlgroup_DiscountRateForDeterminingFairValueOfDepositLiabilities
Namespace Prefix:	xlgroup
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	instant

- Definition

Fair value of certain floating rate medium term notes supported primarily by pools of European credit with varying degrees of leverage that are included within Corporate Fixed Maturity Securities.These notes allow the investor to participate in cash flows of the underlying bonds including certain residual values, which could serve to either decrease or increase the ultimate values of these notes.

+ Details

Name:	xlgroup_FairValueOfFloatingRateMediumTermNotes
Namespace Prefix:	xlgroup
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of equity securities in fixed income maturities.

+ Details

Name:	xlgroup_FixedMaturitiesEquitySecurities
Namespace Prefix:	xlgroup
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The Other investments balance disclosed within the Fair Value Measurements tables exclude certain structured transactions including certain investments in project finance transactions, a payment obligation and liquidity financing provided to a structured credit vehicle as a part of a third party medium term note facility.

+ Details

Name:	xlgroup_InvestmentsInStructuredTransactionsnotCarriedatFairValue
Namespace Prefix:	xlgroup
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant