Premiums and Reinsurance Related Information (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Jun. 30, 2020	Dec. 31, 2019
Written premiums:
Written premiums, direct	$ 2,018,530	$ 1,853,881	$ 5,875,379	$ 5,566,728
Written premiums, assumed	244,015	239,169	750,784	662,412
Written premiums, ceded	(383,229)	(343,144)	(1,161,183)	(1,026,169)
Total net premiums written	1,879,316	1,749,906	5,464,980	5,202,971
Earned premiums:
Earned premiums, direct	1,877,234	1,797,726	5,511,791	5,294,773
Earned premiums, assumed	237,815	209,094	699,025	595,946
Earned premiums, ceded	(366,128)	(330,010)	(1,093,563)	(974,212)
Net premiums earned	1,748,921	1,676,810	5,117,253	4,916,507
Ceded losses and loss expenses incurred	209,706	172,009	677,761	584,323
Ceded commissions earned	89,104	$ 76,001	255,742	$ 220,110
Cumulative effect adjustment resulting from changes in accounting principles	$ 7,741		$ 7,741		$ 7,175	$ 690