Taxation - Components of deferred tax assets and liabilities (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Deferred tax assets:
Net unpaid loss reserve discount	$ 90,262	$ 105,511
Net unearned premiums	72,752	69,811
Compensation liabilities	77,578	62,710
Net operating loss	49,834	89,233
Investment adjustments	13,022us-gaap_DeferredTaxAssetsInvestments	12,573us-gaap_DeferredTaxAssetsInvestments
Pension	8,114	5,695
Bad debt reserve	6,626	7,337
Amortizable goodwill	6,694	7,976
Net unrealized depreciation on investments	1,484	9,984
Stock options	11,985	12,171
Depreciation	7,599	3,501
Net realized capital losses	97,189	140,993
Deferred intercompany capital losses	57,427	69,500
Untaxed Lloyd's result	7,254	2,533
Deferred acquisition costs	11,386	0
Other	2,282us-gaap_DeferredTaxAssetsOther	9,638us-gaap_DeferredTaxAssetsOther
Deferred tax asset, gross of valuation allowance	521,488us-gaap_DeferredTaxAssetsGross	609,166us-gaap_DeferredTaxAssetsGross
Valuation allowance	207,062	261,924
Deferred tax asset, net of valuation allowance	314,426	347,242
Deferred tax liabilities:
Net unrealized appreciation on investments	63,642	28,843
Unremitted earnings	5,125	6,717
Deferred acquisition costs	2,737	7,057
Currency translation adjustments	3,969	7,195
Regulatory reserves	65,965	131,750
Investment adjustment	12,061	6,112
Untaxed Lloyd's result	11,422	0
Other	11,260us-gaap_DeferredTaxLiabilitiesOther	8,014us-gaap_DeferredTaxLiabilitiesOther
Deferred tax liability	176,181xlgroup_DeferredTaxLiabilitiesGross	195,688xlgroup_DeferredTaxLiabilitiesGross
Net deferred tax assets	$ 138,245us-gaap_DeferredTaxAssetsLiabilitiesNet	$ 151,554us-gaap_DeferredTaxAssetsLiabilitiesNet

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Amount before allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences from investments (excludes investments in subsidiaries and equity method investments).

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Amount before allocation of valuation allowances of deferred tax asset attributable to deductible operating loss carryforwards.

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Amount before allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences not separately disclosed.

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Amount before allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences from property, plant, and equipment.

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Amount before allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences from pension benefits.

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Amount before allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences from share-based compensation.

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Amount before allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences from the allowance for doubtful accounts.

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Amount before allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences from loss reserves other than estimated credit losses.

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Amount before allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences from the unearned premiums reserve.

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Amount before allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences from unrealized losses on available-for-sale securities.

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Amount of deferred tax assets for which it is more likely than not that a tax benefit will not be realized.

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Amount of deferred tax liability attributable to taxable temporary differences from other capitalized costs not separately disclosed.

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Amount of deferred tax liability attributable to taxable temporary differences from investments (excludes investments in subsidiaries and equity method investments).

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Amount of deferred tax liability attributable to taxable temporary differences from regulatory assets and liabilities.

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Amount of deferred tax liability attributable to taxable temporary differences from undistributed earnings of subsidiaries and other recognized entities not within the country of domicile of the entity.

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Amount of deferred tax liability attributable to taxable temporary differences from unrealized gains on foreign currency transactions.

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The tax effect as of the balance sheet date of the amount of the estimated future tax deductions attributable to amortizable goodwill.

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The amount as of the balance sheet date of the estimated future tax benefits attributable to policy acquisition costs.

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The tax effect as of the balance sheet date of the amount of the estimated future tax deductions attributable to deferred intercompany capital losses which can only be realized if sufficient taxable income is generated in future periods to enable the deduction to be taken.

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Amount after allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences and carryforwards.

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The amount as of the balance sheet date of the estimated future tax effects attributable to the difference between the tax basis and the basis in accordance with generally accepted accounting principles of untaxed Lloyd's result which will decrease future taxable income when such basis difference reverses.

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Amount before allocation of valuation allowances of deferred tax liability to taxable temporary differences.

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Deferred Tax Liabilities, Investment Adjustment

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