Premiums and Reinsurance Related Information (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Mar. 31, 2020	Dec. 31, 2019
Written premiums:
Written premiums, direct	$ 1,892,359	$ 1,883,032	$ 3,856,848	$ 3,712,847
Written premiums, assumed	239,887	206,828	506,769	423,243
Written premiums, ceded	(392,428)	(346,396)	(777,953)	(683,025)
Total net premiums written	1,739,818	1,743,464	3,585,664	3,453,065
Earned premiums:
Earned premiums, direct	1,804,844	1,773,437	3,634,558	3,497,047
Earned premiums, assumed	232,996	198,661	461,210	386,852
Earned premiums, ceded	(360,925)	(325,257)	(727,435)	(644,202)
Net premiums earned	1,676,915	1,646,841	3,368,333	3,239,697
Ceded losses and loss expenses incurred	232,873	239,267	468,055	412,314
Ceded commissions earned	85,593	$ 73,092	166,638	$ 144,109
Cumulative effect adjustment resulting from changes in accounting principles	$ 7,175		$ 7,175		$ 6,800	$ 690