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Reserve for Loss and Loss Expense (Reserve Loss Development) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Favorable or adverse loss development	$ 72,932	$ 50,289	$ 29,863
Favorable or unfavorable loss development [Member]
Favorable or adverse loss development	(72,900)	(50,300)	(29,900)
Favorable or unfavorable loss development [Member] | Casualty Insurance Product Line [Member]
Favorable or adverse loss development	(85,000)	(61,000)	(41,500)
Favorable or unfavorable loss development [Member] | Casualty Insurance Product Line [Member] | Commercial Automobile [Member]
Favorable or adverse loss development	10,000		37,500
Favorable or unfavorable loss development [Member] | Property Insurance [Member]
Favorable or adverse loss development	12,100	10,700	11,600
Favorable or unfavorable loss development [Member] | Property Insurance [Member] | Commercial Automobile [Member]
Favorable or adverse loss development	7,100
Favorable or unfavorable loss development [Member] | General Liability [Member]
Favorable or adverse loss development	(35,000)	(5,000)	(9,500)
Favorable or unfavorable loss development [Member] | Commercial Automobile [Member]
Favorable or adverse loss development	7,100	700	36,700
Favorable or unfavorable loss development [Member] | Workers Compensation [Member]
Favorable or adverse loss development	(60,000)	(68,000)	(83,000)
Favorable or unfavorable loss development [Member] | Business Owners' Policies [Member]
Favorable or adverse loss development	3,900	1,900	(1,500)
Favorable or unfavorable loss development [Member] | Commercial Property [Member]
Favorable or adverse loss development	9,200	5,100	7,500
Favorable or unfavorable loss development [Member] | Homeowners [Member]
Favorable or adverse loss development	7,700	7,500	9,800
Favorable or unfavorable loss development [Member] | Personal Automobile [Member]
Favorable or adverse loss development	(1,800)	4,400	3,000
Favorable or unfavorable loss development [Member] | E&S Casualty Lines [Member]
Favorable or adverse loss development	0	2,000	12,000
Favorable or unfavorable loss development [Member] | E&S Property Lines [Member]
Favorable or adverse loss development	(4,000)	1,000	(4,800)
Favorable or unfavorable loss development [Member] | Insurance, Other [Member]
Favorable or adverse loss development	$ 0	$ 100	$ (100)

X

- 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

+ Details

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