Acquisition and Disposals - Catlin - Transaction related costs (Details) $ in Thousands	6 Months Ended
Jun. 30, 2017 USD ($)
Business Combination, Separately Recognized Transactions [Line Items]
Liabilities at December 31, 2016	$ 36,966
Costs incurred in 2017	73,067
2017 payments	56,699
Liabilities at June 30, 2017	53,334
Severance related costs
Business Combination, Separately Recognized Transactions [Line Items]
Liabilities at December 31, 2016	25,360
Costs incurred in 2017	19,794
2017 payments	21,929
Liabilities at June 30, 2017	23,225
Retention and other compensation costs
Business Combination, Separately Recognized Transactions [Line Items]
Liabilities at December 31, 2016	4,481
Costs incurred in 2017	9,561
2017 payments	12,369
Liabilities at June 30, 2017	1,673
Facilities-related costs
Business Combination, Separately Recognized Transactions [Line Items]
Liabilities at December 31, 2016	18
Costs incurred in 2017	11,051
2017 payments	2,730
Liabilities at June 30, 2017	8,339
Consulting and other
Business Combination, Separately Recognized Transactions [Line Items]
Liabilities at December 31, 2016	7,107
Costs incurred in 2017	32,661
2017 payments	19,671
Liabilities at June 30, 2017	$ 20,097

Costs incurred to effect a business combination which have been expensed during the period. Such costs could include business integration costs, systems integration and conversion costs, and severance and other employee-related costs.

No definition available.

+ Details

Name:	us-gaap_BusinessCombinationIntegrationRelatedCosts
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The amount of liabilities recognized recorded in a transaction with the acquiree for each transaction that is recognized separately from the acquisition of assets and assumptions of liabilities in the business combination.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (e)(3)
-URI http://asc.fasb.org/extlink&oid=65885119&loc=d3e1392-128463

+ Details

Name:	us-gaap_BusinessCombinationSeparatelyRecognizedTransactionsLiabilitiesRecognized
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

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

- Definition

Amount of cash payments made as the result of exit or disposal activities. Excludes payments associated with a discontinued operation or an asset retirement obligation.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 420
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (b)(2)
-URI http://asc.fasb.org/extlink&oid=6394359&loc=d3e17939-110869

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 25
-URI http://asc.fasb.org/extlink&oid=66902209&loc=d3e3536-108585

+ Details

Name:	us-gaap_PaymentsForRestructuring
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

Name:	us-gaap_RestructuringCostAndReserveAxis=us-gaap_EmployeeSeveranceMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RestructuringCostAndReserveAxis=xlgroup_RetentionCostsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RestructuringCostAndReserveAxis=xlgroup_FacilitiesRelatedCostsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RestructuringCostAndReserveAxis=xlgroup_ConsultingandOtherMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: