Premiums and Reinsurance Related Information (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022	Jun. 30, 2023	Dec. 31, 2022	Jun. 30, 2022	Dec. 31, 2021
Written premiums:
Written premiums, direct	$ 3,037,932	$ 2,775,236	$ 8,762,249	$ 8,075,311
Written premiums, assumed	315,273	306,702	977,047	918,864
Written premiums, ceded	(504,746)	(504,664)	(1,504,497)	(1,418,012)
Total net premiums written	2,848,459	2,577,274	8,234,799	7,576,163
Earned premiums:
Earned premiums, direct	2,825,176	2,617,680	8,217,888	7,533,856
Earned premiums, assumed	316,134	318,962	913,795	903,882
Earned premiums, ceded	(499,396)	(494,681)	(1,445,610)	(1,389,533)
Net premiums earned	2,641,914	2,441,961	7,686,073	7,048,205
Ceded losses and loss expenses incurred	320,995	347,439	970,267	905,667
Ceded commissions earned	118,207	121,721	354,148	356,157
Cumulative effect adjustment resulting from changes in accounting principles	$ 9,443	$ 7,999	$ 9,443	$ 7,999	$ 9,365	$ 8,064	$ 7,744	$ 7,713