Reserve for Claims and Claim Expenses (Schedule of Liability for Unpaid Claims and Claims Adjustment Expense) (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Net reserves as of January 1			$ 3,493,778	$ 2,568,730
Net incurred related to:
Current year	$ 419,926	$ 1,209,261	839,360	1,561,027
Prior years	(9,416)	12,435	(196,980)	(3,663)
Total net incurred	410,510	1,221,696	642,380	1,557,364
Net paid related to:
Current year			48,075	158,685
Prior years			326,692	396,411
Total net paid			374,767	555,096
Foreign exchange			(12,952)	33,011
Net reserves as of September 30	3,748,439	3,604,009	3,748,439	3,604,009
Reinsurance recoverable as of September 30	1,204,059	1,588,304	1,204,059	1,588,304
Gross reserves as of September 30	$ 4,952,498	$ 5,192,313	$ 4,952,498	$ 5,192,313

The amount needed to reflect the estimated ultimate cost of settling claims relating to insured events that have occurred on or before the balance sheet date, whether or not reported to the insurer at that date.

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Amount, after effects of reinsurance, of payments to settle claims incurred in the current period and related claims settlement costs.

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Amount, after effects of reinsurance, of payments to settle claims incurred in prior periods and related claims settlement costs.

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Amount of foreign currency gain (loss) which decreases (increases) the cost of settling unpaid claims after deduction of reinsurance recoveries. Includes, but is not limited to, claims which have been incurred but not reported.

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Amount, after effects of reinsurance, of expense (reversal of expense) for claims incurred and costs incurred in the claim settlement process.

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Liability as of the balance sheet date for amounts representing estimated cost of settling unpaid claims under the terms of the underlying insurance policies, less estimated reinsurance recoveries on such claims. This includes an estimate for claims which have been incurred but not reported. Claim adjustment expenses represent the costs estimated to be incurred in the settlement of unpaid claims.

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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Amount, after effects of reinsurance, of payments to settle insured claims and pay costs incurred in the claims settlement process.

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Amount, after valuation allowance, recoverable under reinsurance contracts including premium paid under reinsurance contracts. Examples include, but are not limited to, settled and unsettled claims, incurred but not reported losses, loss adjustment expense, premium paid, policy benefits and policy reserves.

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Amount, after effects of reinsurance, of expense for claims incurred in the current reporting period and related claims settlement costs.

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Amount, after effects of reinsurance, of expense (reversal of expense) for claims incurred in prior reporting periods and related claims settlement costs.

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