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Reinsurance (List of Direct, Assumed, and Ceded Reinsurance Amounts) (Details) - USD ($) $ in Thousands	3 Months Ended				9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2021	Sep. 30, 2020
Premiums written:
Change in Direct Premiums Written					$ 375,900
Change in Direct Premiums Written Percentage					16.00%
Premiums earned:
Net	$ 767,247	$ 694,541			$ 2,232,725	$ 1,976,915
Loss and loss expenses incurred:
Net	$ 505,269	447,802			1,340,293	1,252,075
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	4.00%
Direct Assumed and Ceded Reinsurance Amounts [Member]
Premiums written:
Direct	$ 932,752	839,275			2,796,296	2,420,350
Assumed	7,136	5,450			17,541	17,902
Ceded	(126,982)	(125,217)			(369,548)	(346,665)
Net	812,906	719,508			2,444,289	2,091,587
Premiums earned:
Direct	877,620	803,957			2,568,445	2,292,495
Assumed	6,304	6,147			16,391	18,375
Ceded	(116,677)	(115,563)			(352,111)	(333,955)
Net	767,247	694,541			2,232,725	1,976,915
Loss and loss expenses incurred:
Direct	655,483	530,192			1,557,063	1,407,000
Assumed	4,143	4,232			10,807	13,430
Ceded	(154,357)	(86,622)			(227,577)	(168,355)
Net	$ 505,269	$ 447,802			$ 1,340,293	$ 1,252,075

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