Premiums and Reinsurance Related Information (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021	Mar. 31, 2022	Dec. 31, 2021	Mar. 31, 2021	Dec. 31, 2020
Written premiums:
Written premiums, direct	$ 2,757,739	$ 2,395,114	$ 5,300,075	$ 4,572,276
Written premiums, assumed	294,662	266,122	612,163	573,672
Written premiums, ceded	(466,766)	(449,055)	(913,349)	(883,729)
Total net premiums written	2,585,635	2,212,181	4,998,889	4,262,219
Earned premiums:
Earned premiums, direct	2,511,579	2,151,739	4,916,177	4,154,784
Earned premiums, assumed	292,547	259,166	584,920	518,707
Earned premiums, ceded	(446,968)	(439,281)	(894,853)	(851,911)
Net premiums earned	2,357,158	1,971,624	4,606,244	3,821,580
Ceded losses and loss expenses incurred	305,196	255,163	548,490	553,903
Ceded commissions earned	116,991	106,834	234,436	208,515
Cumulative effect adjustment resulting from changes in accounting principles	$ 7,744	$ 7,283	$ 7,744	$ 7,283	$ 7,655	$ 7,713	$ 7,373	$ 7,801