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Reserve for Loss and Loss Expense (Reserve Loss Development) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Prior Years	$ 753,321	$ 742,722	$ 670,176
Favorable or adverse loss development	(29,863)	(39,192)	(65,768)
Favorable or unfavorable loss development [Member]
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Prior Years	29,900	39,200	65,800
Favorable or adverse loss development	(29,900)	(39,200)	(65,800)
Favorable or unfavorable loss development [Member] | Casualty Insurance Product Line [Member]
Favorable or adverse loss development	41,500	48,600	69,000
Favorable or unfavorable loss development [Member] | Property Insurance Product Line [Member]
Favorable or adverse loss development	11,600	9,400	3,200
Favorable or unfavorable loss development [Member] | General Liability [Member]
Favorable or adverse loss development	(9,500)	(48,300)	(45,000)
Favorable or unfavorable loss development [Member] | Commercial Automobile [Member]
Favorable or adverse loss development	36,700	35,600	25,300
Casualty Development	37,500	36,000	25,000
Favorable or unfavorable loss development [Member] | Workers Compensation [Member]
Favorable or adverse loss development	(83,000)	(52,300)	(56,000)
Favorable or unfavorable loss development [Member] | Business Owners' Policies [Member]
Favorable or adverse loss development	(1,500)	1,900	1,800
Favorable or unfavorable loss development [Member] | Commercial Property [Member]
Favorable or adverse loss development	7,500	8,700	300
Favorable or unfavorable loss development [Member] | Homeowners [Member]
Favorable or adverse loss development	9,800	400	1,700
Favorable or unfavorable loss development [Member] | Personal Automobile [Member]
Favorable or adverse loss development	3,000	6,700	1,000
Favorable or unfavorable loss development [Member] | E&S Casualty Lines [Member]
Favorable or adverse loss development	12,000	10,000	6,000
Favorable or unfavorable loss development [Member] | E&S Property Lines [Member]
Favorable or adverse loss development	(4,800)	100	1,200
Favorable or unfavorable loss development [Member] | Other Insurance Product Line [Member]
Favorable or adverse loss development	$ (100)	$ (2,000)	$ (2,100)

X

- Definition

Amount of provision in the period for casualty claims incurred in prior reporting periods and related claims settlement costs.

+ References

No definition available.

+ Details

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- Definition

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

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 40
-Section 50
-Paragraph 3
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=99382578&loc=d3e14764-158437

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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
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-URI http://asc.fasb.org/extlink&oid=99382578&loc=d3e14764-158437

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