Quarterly Financial Information (Summary Of Quarterly Financial Information) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended													12 Months Ended
	Dec. 31, 2015		Sep. 30, 2015		Jun. 30, 2015		Mar. 31, 2015		Dec. 31, 2014		Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Quarterly Financial Data [Abstract]
Net premiums earned	$ 760,094		$ 745,520		$ 731,546		$ 720,737		$ 709,368		$ 705,237	$ 697,889	$ 683,701	$ 2,957,897	$ 2,796,195	$ 2,698,187
Change in fair value of investments pursuant to the fair value option	2,474		(18,538)		(40,783)		(4,884)		(22,824)		(20,528)	41,412	45,699	(61,731)	43,759	(44,037)
Income before income taxes	23,930		12,267		4,511		29,859		(28,161)		37,120	136,436	102,030	70,567	247,425	132,096
Net income	$ 23,405		$ 15,270		$ 9,639		$ 26,165		$ (20,956)		$ 31,296	$ 94,960	$ 72,649	$ 74,479	$ 177,949	$ 112,143
Basic (in dollars per share)	$ 0.42	[1]	$ 0.28	[1]	$ 0.17	[1]	$ 0.47	[1]	$ (0.38)		$ 0.57	$ 1.73	$ 1.32	$ 1.35	$ 3.23	$ 2.04
Diluted (in dollars per share)	0.42	[1]	0.28	[1]	0.17	[1]	0.47	[1]	(0.38)	[1]	0.57	1.73	1.32	1.35	3.23	$ 2.04
Dividends paid per share	$ 0.62		$ 0.6175		$ 0.6175		$ 0.6175		$ 0.6175		$ 0.6175	$ 0.6150	$ 0.6150	$ 0.62	$ 0.6175

[1] The basic and diluted earnings per share do not sum due to rounding.