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Reinsurance (List of Direct, Assumed, and Ceded Reinsurance Amounts) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Premiums written:
Change in Direct Premiums Written	$ 162,000
Change in Direct Premiums Written Percentage	22.00%
Premiums earned:
Net	$ 724,960	$ 651,703
Loss and loss expenses incurred:
Net	$ 413,401	400,324
Return audit and midterm endorsement premium [Member] | COVID-19 [Member]
Premiums written:
Direct		75,000
Change in Direct Premiums Written Percentage	11.00%
Change in Direct Premiums Written
Premiums written:
Direct	$ 87,000
Direct Assumed and Ceded Reinsurance Amounts [Member]
Premiums written:
Direct	908,774	746,431
Assumed	5,533	6,043
Ceded	(116,129)	(105,147)
Net	798,178	647,327
Premiums earned:
Direct	837,369	754,892
Assumed	5,676	6,173
Ceded	(118,085)	(109,362)
Net	724,960	651,703
Loss and loss expenses incurred:
Direct	441,507	425,795
Assumed	3,447	4,898
Ceded	(31,553)	(30,369)
Net	$ 413,401	$ 400,324

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