Quarterly Financial Information (Unaudited) (Summary Of Quarterly Financial Information) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Quarterly Financial Data [Abstract]
Net premiums earned	$ 921,860	$ 899,304	$ 811,897	$ 922,574	$ 925,384	$ 915,012	$ 888,776	$ 870,245	$ 3,555,635	$ 3,599,418	$ 3,368,411
Change in fair value of financial instruments pursuant to the fair value option	87,968	58,865	166,829	(240,990)	18,173	24,021	50,281	104,227
(Loss) income before income taxes	207,110	146,631	285,466	(180,705)	28,465	80,840	101,595	167,169	458,501	378,069	(30,615)
Net income (loss)	$ 166,743	$ 118,857	$ 228,211	$ (139,204)	$ 31,688	$ 69,282	$ 83,250	$ 135,867	$ 374,607	$ 320,087	$ (5,728)
Basic (loss) earnings per share (in dollars per share)	$ 3.01	$ 2.15	$ 4.12	$ (2.51)	$ 0.57	$ 1.25	$ 1.50	$ 2.46	$ 6.77	$ 5.78	$ (0.10)
Diluted (loss) earnings per share (in dollars per share)	3.01	2.15	4.12	(2.51)	0.57	1.25	1.50	2.45	6.77	5.78	$ (0.10)
Dividends paid per share (in dollars per share)	$ 0.6325	$ 0.6300	$ 0.6300	$ 0.6300	$ 0.6300	$ 0.6275	$ 0.6275	$ 0.6275	$ 0.6325	$ 0.6300