Reinsurance (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Written premiums:
Written premiums, direct	$ 1,964,490	$ 1,829,815
Written premiums, assumed	266,882	216,415
Written premiums, ceded	(385,526)	(336,629)
Total net premiums written	1,845,846	1,709,601
Earned premiums:
Earned premiums, direct	1,829,713	1,723,610
Earned premiums, assumed	228,214	188,191
Earned premiums, ceded	(366,509)	(318,945)
Net premiums earned	1,691,418	1,592,856
Ceded losses and loss expenses incurred	235,183	173,046
Ceded commissions earned	81,045	$ 71,017
Cumulative effect adjustment resulting from changes in accounting principles	$ 6,800		$ 690