Premiums and Reinsurance Related Information (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021	Jun. 30, 2022	Dec. 31, 2021	Jun. 30, 2021	Dec. 31, 2020
Written premiums:
Written premiums, direct	$ 2,775,236	$ 2,490,875	$ 8,075,311	$ 7,063,151
Written premiums, assumed	306,702	296,623	918,864	870,295
Written premiums, ceded	(504,664)	(462,360)	(1,418,012)	(1,346,089)
Total net premiums written	2,577,274	2,325,138	7,576,163	6,587,357
Earned premiums:
Earned premiums, direct	2,617,680	2,276,173	7,533,856	6,430,957
Earned premiums, assumed	318,962	273,808	903,882	792,515
Earned premiums, ceded	(494,681)	(468,963)	(1,389,533)	(1,320,874)
Net premiums earned	2,441,961	2,081,018	7,048,205	5,902,598
Ceded losses and loss expenses incurred	347,439	318,283	905,667	872,186
Ceded commissions earned	121,721	113,523	356,157	322,037
Cumulative effect adjustment resulting from changes in accounting principles	$ 7,999	$ 7,277	$ 7,999	$ 7,277	$ 7,744	$ 7,713	$ 7,283	$ 7,801