Quarterly Financial Information (Unaudited) (Summary Of Quarterly Financial Information) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Quarterly Financial Data [Abstract]
Net premiums earned	$ 794,517	$ 790,850	$ 779,321	$ 767,085	$ 760,094	$ 745,520	$ 731,546	$ 720,737	$ 3,131,773	$ 2,957,897	$ 2,796,195
Change in fair value of investments pursuant to the fair value option	(67,332)	(21,132)	37,127	18,531	2,474	(18,538)	(40,783)	(4,884)	(32,805)	(61,731)	43,759
Income before income taxes	(51,270)	31,625	64,335	26,034	23,930	12,267	4,511	29,859	70,724	70,567	247,425
Net income	$ (26,082)	$ 26,930	$ 48,873	$ 23,323	$ 23,405	$ 15,270	$ 9,639	$ 26,165	$ 73,044	$ 74,479	$ 177,949
Basic (in dollars per share)	$ (0.47)	$ 0.49	$ 0.88	$ 0.42	$ 0.42	$ 0.28	$ 0.17	$ 0.47	$ 1.32	$ 1.35	$ 3.23
Diluted (in dollars per share)	(0.47)	0.49	0.88	0.42	0.42	0.28	0.17	0.47	1.32	1.35	$ 3.23
Dividends paid per share	$ 0.6225	$ 0.6200	$ 0.6200	$ 0.62	$ 0.62	$ 0.6175	$ 0.6175	$ 0.6175	$ 0.6225	$ 0.62