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Reinsurance (List of direct, assumed and ceded Reinsurance Amounts) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Premiums Written:
Direct premiums written									$ 3,204,512	$ 3,084,451	$ 2,890,633
Assumed									24,288	24,339	26,250
Ceded premiums written									(455,708)	(429,366)	(402,597)
Net									2,773,092	2,679,424	2,514,286
Direct premiums written									3,204,512	3,084,451	2,890,633
Premiums Earned:
Direct									3,108,687	2,993,157	2,808,764
Assumed									25,010	24,399	25,831
Ceded premiums earned									(451,883)	(420,385)	(398,366)
Total Net Premiums Earned	$ 704,899	$ 694,541	$ 630,671	$ 651,703	$ 668,359	$ 653,620	$ 642,619	$ 632,573	2,681,814	2,597,171	2,436,229
Loss and loss expense incurred:
Direct									1,822,034	1,714,880	1,706,951
Assumed									17,201	22,879	21,469
Ceded loss and loss expense									(203,412)	(186,268)	(230,286)
Net									1,635,823	1,551,491	$ 1,498,134
Loss from Catastrophes									215,400	$ 81,000
Audit and Endorsement Premium									$ 82,500
Policyholder Premium Credit			15.00%
Increase in DPW from Prior Year									4.00%	7.00%
Return audit and midterm endorsement premium [Member] | COVID-19 [Member]
Premiums Written:
Direct premiums written									$ 75,000
Direct premiums written									75,000
Return audit and midterm endorsement premium [Member] | COVID-19 [Member] | Remaining Accrual [Member]
Premiums Written:
Direct premiums written									24,800
Direct premiums written									24,800
Commercial & Personal Automobile [Member] | COVID-19 [Member]
Premiums Written:
Direct premiums written									19,700
Direct premiums written									$ 19,700

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