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Reinsurance (List of Direct, Assumed, and Ceded Reinsurance Amounts) (Details) - USD ($) $ in Thousands	3 Months Ended			6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2021	Jun. 30, 2020
Premiums written:
Change in Direct Premiums Written	$ 120,100			$ 282,500
Change in Direct Premiums Written Percentage	14.00%			18.00%
Premiums earned:
Net	$ 740,518	$ 630,671		$ 1,465,478	$ 1,282,374
Loss and loss expenses incurred:
Net	$ 421,623	$ 403,949		$ 835,024	804,273
Policyholder Premium Credit		15.00%
COVID-19 [Member] | Commercial & Personal Automobile [Member] | Premium Credit [Member]
Premiums written:
Direct		$ 19,700
Return audit and midterm endorsement premium [Member] | COVID-19 [Member]
Premiums written:
Direct			$ 75,000
Change in Direct Premiums Written Percentage	3.00%			7.00%
Direct Assumed and Ceded Reinsurance Amounts [Member]
Premiums written:
Direct	$ 954,770	834,643		$ 1,863,544	1,581,074
Assumed	4,872	6,410		10,405	12,453
Ceded	(126,437)	(116,301)		(242,566)	(221,448)
Net	833,205	724,752		1,631,383	1,372,079
Premiums earned:
Direct	853,456	733,647		1,690,825	1,488,538
Assumed	4,411	6,055		10,087	12,228
Ceded	(117,349)	(109,031)		(235,434)	(218,392)
Net	740,518	630,671		1,465,478	1,282,374
Loss and loss expenses incurred:
Direct	460,073	451,013		901,580	876,808
Assumed	3,217	4,300		6,664	9,198
Ceded	(41,667)	(51,364)		(73,220)	(81,733)
Net	$ 421,623	$ 403,949		$ 835,024	$ 804,273

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