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Unpaid Losses And Loss Adjustment Expenses (Narrative) (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019
Causes of (Increase) Decrease in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Favorable (adverse) development on prior years' loss reserves	$ 268,220	$ 188,949
Prof liab general liab and workers comp Insurance Segment and prop and general liab Resinsurance Segment [Member]
Causes of (Increase) Decrease in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Favorable (adverse) development on prior years' loss reserves	206,600
Gen liab workers comp and personal lines Insurance segment and aviation whole account and auto Reinsurance segment [Member]
Causes of (Increase) Decrease in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Favorable (adverse) development on prior years' loss reserves		$ 168,100
COVID-19 Pandemic [Member]
Causes of (Increase) Decrease in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net losses and loss adjustment expenses, net of assumed reinstatement premiums	325,000
Reinsurance recoverables	$ 58,000

X

- Definition

Amount, after effects of reinsurance and assumed reinstatement premiums, of expense related to provisions for policy benefits and costs incurred for property and casualty insurance contracts

+ References

No definition available.

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- Definition

Amount of reduction of provision for policy benefits and costs incurred for policies ceded.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 20
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=99397103&loc=d3e6811-158387

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- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 40
-Section 50
-Paragraph 3
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=116884095&loc=d3e14764-158437

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