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Taxation - Narrative (Details) (USD $)	12 Months Ended
Taxation - Narrative (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Tax Disclosure [Line Items]
Deferred tax asset, per balance sheet	$ 204,491,000us-gaap_DeferredTaxAssetsNet	$ 237,884,000us-gaap_DeferredTaxAssetsNet
Deferred tax liability, per balance sheet	66,246,000us-gaap_DeferredIncomeTaxLiabilities	86,330,000us-gaap_DeferredIncomeTaxLiabilities
Valuation allowance	207,062,000	261,924,000
Increase (decrease) in valuation allowance, amount	54,900,000
Net realized capital losses	97,189,000	140,993,000
Excess tax benefits on share-based compensation	4,000,000	1,800,000
Accrued interest and penalties	400,000	1,000,000
Unrecognized tax benefits	37,190,000us-gaap_UnrecognizedTaxBenefits	30,000,000us-gaap_UnrecognizedTaxBenefits	0us-gaap_UnrecognizedTaxBenefits
United States
Income Tax Disclosure [Line Items]
Valuation allowance	123,695,000	175,372,000
Capital loss valuation allowance, amount	61,100,000
Capital loss carryforwards available	174,600,000
Net operating loss carryforwards, tax effected	61,100,000
United States \| Deferred intercompany capital losses
Income Tax Disclosure [Line Items]
Capital loss valuation allowance, amount	57,400,000
Capital loss carryforwards, tax effected	57,400,000
Capital loss carryforwards available	164,100,000
United States \| Expiring in 2015
Income Tax Disclosure [Line Items]
Capital loss carryforwards available	75,600,000
United States \| Expiring in future years
Income Tax Disclosure [Line Items]
Capital loss carryforwards available	99,000,000
Ireland
Income Tax Disclosure [Line Items]
Valuation allowance	72,363,000	60,133,000
Capital loss carryforwards available	153,500,000
Net operating loss carryforwards, amount	280,800,000
Net operating loss carryforwards valuation allowance, amount	69,600,000
United Kingdom
Income Tax Disclosure [Line Items]
Capital loss carryforwards, tax effected	1,600,000
Net operating loss carryforwards, amount	84,500,000
Net operating loss carryforwards valuation allowance, amount	1,600,000
Net operating loss carryforwards, tax effected	1,900,000
Net realized capital losses	8,100,000
United Kingdom \| Uncertain Tax Position [Member]
Income Tax Disclosure [Line Items]
Net operating loss carryforwards, tax effected	15,000,000
United Kingdom \| Excluding Uncertain Tax Position [Member]
Income Tax Disclosure [Line Items]
Net operating loss carryforwards, tax effected	16,900,000
United Kingdom \| Expected to be used
Income Tax Disclosure [Line Items]
Net operating loss carryforwards, amount	84,500,000
Switzerland
Income Tax Disclosure [Line Items]
Valuation allowance	1,459,000	11,457,000
Net operating loss carryforwards, amount	12,100,000
Net operating loss carryforwards valuation allowance, amount	800,000
Switzerland \| Not expected to be utilized in the future
Income Tax Disclosure [Line Items]
Net operating loss carryforwards, amount	$ 11,700,000