Premiums and Reinsurance Related Information (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020	Dec. 31, 2019
Written premiums:
Written premiums, direct	$ 2,177,162	$ 1,964,490
Written premiums, assumed	307,550	266,882
Written premiums, ceded	(434,674)	(385,526)
Total net premiums written	2,050,038	1,845,846
Earned premiums:
Earned premiums, direct	2,003,045	1,829,713
Earned premiums, assumed	259,541	228,214
Earned premiums, ceded	(412,630)	(366,509)
Net premiums earned	1,849,956	1,691,418
Ceded losses and loss expenses incurred	298,740	235,183
Ceded commissions earned	101,680	81,045
Cumulative effect adjustment resulting from changes in accounting principles	$ 7,373	$ 6,800	$ 7,801	$ 690