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, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Net reserves as of beginning of period			$ 3,704,050	$ 3,493,778
Net incurred related to:
Current year	$ 664,967	$ 419,926	1,349,448	839,360
Prior years	(10,447)	(9,416)	(14,520)	(196,980)
Total net incurred	654,520	410,510	1,334,928	642,380
Net paid related to:
Current year			128,471	48,075
Prior years			563,539	326,692
Total net paid			692,010	374,767
Amounts acquired			1,858,775	0
Foreign exchange			(41,605)	(12,952)
Net reserves as of end of period	6,164,138	3,748,439	3,704,050	3,493,778
Reinsurance recoverable as of end of period	2,438,299	1,204,059	2,438,299	1,204,059
Gross reserves as of end of period	$ 8,602,437	$ 4,952,498	$ 8,602,437	$ 4,952,498

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 of increase (decrease) in the cost of settling unpaid claims after deduction of reinsurance recoveries related to the acquisition of a business. Includes, but is not limited to, claims which have been incurred but not reported.

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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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