Investments - Analysis of net investment income (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Net Investment Income [Line Items]
Equity securities and other investments	$ 42,252us-gaap_InvestmentIncomeDividend	$ 35,388us-gaap_InvestmentIncomeDividend	$ 29,807us-gaap_InvestmentIncomeDividend
Funds withheld	14,583xlgroup_InvestmentIncomeFundsWithheld	12,783xlgroup_InvestmentIncomeFundsWithheld	12,090xlgroup_InvestmentIncomeFundsWithheld
Total gross investment income	996,374	1,030,446	1,088,152
Investment expenses	(77,749)	(72,730)	(75,804)
Net investment income	918,625us-gaap_InvestmentIncomeNet	957,716us-gaap_InvestmentIncomeNet	1,012,348us-gaap_InvestmentIncomeNet
Exclude Life Funds Withheld Assets [Member]
Net Investment Income [Line Items]
Fixed maturities, short term investments and cash equivalents	809,964	982,275	1,046,255
Net investment income	789,050	957,716	1,012,348
Life Funds Withheld Assets [Member]
Net Investment Income [Line Items]
Fixed maturities, short term investments and cash equivalents	129,575	0	0
Net investment income	$ 129,575	$ 0	$ 0

Amount of dividend income on nonoperating securities.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 7

-Article 5

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 225

-SubTopic 10

-Section S99

-Paragraph 2

-Subparagraph (SX 210.5-03.7(a))

-URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688

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Amount before accretion (amortization) of purchase discount (premium) of interest income on nonoperating securities.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 7

-Article 5

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 225

-SubTopic 10

-Section S99

-Paragraph 2

-Subparagraph (SX 210.5-03.7(b))

-URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688

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Amount before accretion (amortization) of purchase discount (premium) of interest income and dividend income on nonoperating securities.

+ References

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Amount of expenses related to the generation of investment income.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 225

-Section S99

-Paragraph 1

-Subparagraph (SX 210.7-04.2)

-URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 225

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-Section S99

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-URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688

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Amount after accretion (amortization) of discount (premium), and investment expense, of interest income and dividend income on nonoperating securities.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 225

-SubTopic 10

-Section S99

-Paragraph 2

-Subparagraph (SX 210.5-03.8)

-URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 225

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-Subparagraph (SX 210.5-03.7(a),(b))

-URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 225

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-Section S99

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Income derived from investments in funds withheld distribution of earnings to shareholders by the investee companies.

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