Premiums and Reinsurance Related Information (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020	Dec. 31, 2019
Written premiums:
Written premiums, direct	$ 2,395,114	$ 1,892,359	$ 4,572,276	$ 3,856,848
Written premiums, assumed	266,122	239,887	573,672	506,769
Written premiums, ceded	(449,055)	(392,428)	(883,729)	(777,953)
Total net premiums written	2,212,181	1,739,818	4,262,219	3,585,664
Earned premiums:
Earned premiums, direct	2,151,739	1,804,844	4,154,784	3,634,558
Earned premiums, assumed	259,166	232,996	518,707	461,210
Earned premiums, ceded	(439,281)	(360,925)	(851,911)	(727,435)
Net premiums earned	1,971,624	1,676,915	3,821,580	3,368,333
Ceded losses and loss expenses incurred	255,163	232,873	553,903	468,055
Ceded commissions earned	106,834	85,593	208,515	166,638
Cumulative effect adjustment resulting from changes in accounting principles	$ 7,283	$ 7,175	$ 7,283	$ 7,175	$ 7,373	$ 7,801	$ 6,800	$ 690