Notes Payable and Debt and Financing Arrangements - Letter of credit facilities and other sources of collateral (Details) $ in Thousands	Mar. 31, 2018 USD ($) Credit_Facilities	Dec. 31, 2017 USD ($) Credit_Facilities
Line of Credit Facility [Line Items]
Number of letter of credit facilities available \| Credit_Facilities	10	10
Letter of credit
Line of Credit Facility [Line Items]
Available letter of credit facilities - commitments	$ 3,467,841	$ 3,392,986
Available letter of credit facilities - in use	$ 2,807,211	$ 2,549,226
Collateralized by certain assets of the Company’s investment portfolio	54.70%	54.20%
Revolving credit facility
Line of Credit Facility [Line Items]
Available credit facilities - remaining borrowing capacity	$ 644,300	$ 748,000
Available letter of credit facilities - commitments	750,000
Available letter of credit facilities - in use	105,700	2,000
Credit Agricole Facility I [Member] \| Letter of credit
Line of Credit Facility [Line Items]
Available letter of credit facilities - commitments	125,000	125,000
Available letter of credit facilities - in use	125,000	125,000
Credit Agricole Facility II [Member] \| Letter of credit
Line of Credit Facility [Line Items]
Available letter of credit facilities - commitments	125,000	125,000
Available letter of credit facilities - in use	125,000	125,000
Commonwealth Bank Facility [Member] \| Letter of credit
Line of Credit Facility [Line Items]
Available letter of credit facilities - commitments	215,000	215,000
Available letter of credit facilities - in use	215,000	215,000
Credit Suisse Facility [Member] \| Letter of credit
Line of Credit Facility [Line Items]
Available letter of credit facilities - commitments	100,000	100,000
Available letter of credit facilities - in use	100,000	100,000
Funds at Lloyds Bilateral Facility I [Member] \| Letter of credit
Line of Credit Facility [Line Items]
Available letter of credit facilities - commitments	125,000	125,000
Available letter of credit facilities - in use	125,000	125,000
Funds at Lloyds Bilateral Facility II [Member] \| Letter of credit
Line of Credit Facility [Line Items]
Available letter of credit facilities - commitments	125,000	125,000
Available letter of credit facilities - in use	125,000	125,000
Funds at Lloyds Bilateral Facility III [Member] \| Letter of credit
Line of Credit Facility [Line Items]
Available letter of credit facilities - commitments	125,000	125,000
Available letter of credit facilities - in use	125,000	125,000
Funds at Lloyds Bilateral Facility IV [Member] \| Letter of credit
Line of Credit Facility [Line Items]
Available letter of credit facilities - commitments	125,000	125,000
Available letter of credit facilities - in use	125,000	125,000
Syndicated Lenders [Member] \| Unsecured Debt [Member]
Line of Credit Facility [Line Items]
Available letter of credit facilities - commitments	750,000	750,000
Available letter of credit facilities - in use	105,655	2,000
Commerzbank Facility [Member] \| Letter of credit
Line of Credit Facility [Line Items]
Available letter of credit facilities - commitments	100,000	100,000
Available letter of credit facilities - in use	100,000	100,000
Unsecured Credit Facilities [Member] \| Letter of credit
Line of Credit Facility [Line Items]
Available letter of credit facilities - commitments	1,915,000	1,915,000
Available letter of credit facilities - in use	1,270,655	1,167,000
Secured Credit Facilities [Member] \| Letter of credit
Line of Credit Facility [Line Items]
Available letter of credit facilities - commitments	1,552,841	1,477,986
Available letter of credit facilities - in use	$ 1,536,556	$ 1,382,226

The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(16))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

+ Details

Name:	us-gaap_LineOfCredit
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ References

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of borrowing capacity currently available under the credit facility (current borrowing capacity less the amount of borrowings outstanding).

+ References

+ Details

Name:	us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Number of letter of credit facilities available.

+ References

No definition available.

+ Details

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Namespace Prefix:	xlgroup_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Percentage of Letters of credit collateralized by Investments.

+ References

No definition available.

+ Details

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Namespace Prefix:	xlgroup_
Data Type:	num:percentItemType
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