Commitments, Contingencies and Other Items (Narrative) (Details) - 12 months ended Dec. 31, 2015 £ in Thousands	USD ($)	GBP (£)
Commitments and Contingencies [Line Items]
Margin facility amount outstanding	$ 712,126,000	£ 85,000
Initial commitments	724,500,000
Fulfilled commitments	528,800,000
Unfunded commitments	$ 204,500,000
Initial capital lease term	20 years
Bargain renewal option term	30 years
Reduction in Taxes \| Catastrophe Reinsurance
Commitments and Contingencies [Line Items]
Recovery of previously recognized acquisition expenses	$ 7,700,000
Letter of Credit \| Syndicate 1458 \| Citibank Europe PLC
Commitments and Contingencies [Line Items]
Letters of credit outstanding	360,000,000	£ 85,000
Letter of Credit
Commitments and Contingencies [Line Items]
Letters of credit outstanding	837,400,000
Letter of Credit \| Top Layer Reinsurance Ltd.
Commitments and Contingencies [Line Items]
Collateral pledged to support letter of credit	37,500,000
Letter of Credit \| Capital Support Agreement \| Top Layer Reinsurance Ltd.
Commitments and Contingencies [Line Items]
Maximum mandatory capital contribution in the event of loss that reduced capital and surplus below specified level	$ 50,000,000
Maximum \| Investments as a Percentage of Shareholders' Equity
Commitments and Contingencies [Line Items]
Concentration risk percentage	10.00%

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Capital Lease, Term, Renewal Option

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Commitments and Contingencies [Line Items]

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Private Equity Partnerships and Other Entities, Commitments Fulfilled

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Private Equity Partnerships and Other Entities, Initial Commitments

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Private Equity Partnerships and Other Entities, Unfunded Commitments

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For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

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Amount of gain recognized in the period that had been disclosed in prior periods as a contingent gain.

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The total amount of the contingent obligation under letters of credit outstanding as of the reporting date.

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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.

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The estimated maximum amount of possible loss from the contingency.

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The carrying amount, as of the date of the latest financial statement presented, of all assets that are pledged as collateral for the payment of the related debt obligation primarily arising from secured borrowings, repurchase agreements or other current or potential obligations, where the secured party does not have the right by contract or custom to sell or re-pledge the collateral and therefore are not reclassified and separately reported in the statement of financial position.

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