Reinsurance (List of Direct, Assumed, and Ceded Reinsurance Amounts) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Premiums earned:
Net	$ 694,541	$ 653,620	$ 1,976,915	$ 1,928,812
Loss and loss expenses incurred:
Net	447,802	398,675	1,252,075	1,166,238
Loss from Catastrophes	79,500	24,200	195,900	74,500
Direct Assumed and Ceded Reinsurance Amounts [Member]
Premiums written:
Direct	839,275	785,680	2,420,350	2,359,441
Assumed	5,450	6,882	17,902	18,770
Ceded	(125,217)	(115,634)	(346,665)	(326,939)
Net	719,508	676,928	2,091,587	2,051,272
Premiums earned:
Direct	803,957	752,872	2,292,495	2,221,257
Assumed	6,147	6,356	18,375	18,660
Ceded	(115,563)	(105,608)	(333,955)	(311,105)
Net	694,541	653,620	1,976,915	1,928,812
Loss and loss expenses incurred:
Direct	530,192	437,617	1,407,000	1,297,975
Assumed	4,232	4,362	13,430	13,975
Ceded	(86,622)	(43,304)	(168,355)	(145,712)
Net	$ 447,802	$ 398,675	$ 1,252,075	$ 1,166,238