Quarterly Financial Information (Unaudited) (Summary Of Quarterly Financial Information) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Quarterly Financial Data [Abstract]
Net premiums earned	$ 925,384	$ 915,012	$ 888,776	$ 870,245	$ 868,233	$ 858,135	$ 833,959	$ 808,084	$ 3,599,418	$ 3,368,411	$ 3,195,437
Change in fair value of financial instruments pursuant to the fair value option	18,173	24,021	50,281	104,227	(70,020)	(12,907)	8,793	(58,532)	196,594	(132,658)	87,927
Income before income taxes	28,465	80,840	101,595	167,169	(114,448)	70,286	73,246	(59,699)	378,069	(30,615)	167,085
Net income (loss)	$ 31,688	$ 69,282	$ 83,250	$ 135,867	$ (81,879)	$ 58,578	$ 60,180	$ (42,607)	$ 320,087	$ (5,728)	$ 144,877
Basic (loss) earnings per share (in dollars per share)	$ 0.57	$ 1.25	$ 1.50	$ 2.46	$ (1.48)	$ 1.06	$ 1.09	$ (0.77)	$ 5.78	$ (0.10)	$ 2.62
Diluted (loss) earnings per share (in dollars per share)	0.57	1.25	1.50	2.45	(1.48)	1.06	1.09	(0.77)	5.78	(0.10)	$ 2.62
Dividends paid per share (in dollars per share)	$ 0.6300	$ 0.6275	$ 0.6275	$ 0.6275	$ 0.6275	$ 0.6250	$ 0.6250	$ 0.6250	$ 0.6300	$ 0.6275