Quarterly Financial Information (Summary Of Quarterly Financial Information) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended										12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011		Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010		Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Net premiums earned	$ 641,613	$ 643,626		$ 642,331	$ 638,487	$ 640,796		$ 642,558	$ 642,717	$ 640,614
Change in fair value of investments pursuant to the fair value option	54,100	(64,312)		20,597	20,904	(29,469)		87,647	(30,537)	18,939
Income (loss) before income taxes	110,693	(18,118)		75,613	76,911	(50,097)		135,839	15,358	81,290	245,099	182,390	571,541
Net income	79,469	(3,782)		57,251	58,226	(23,647)		96,849	17,817	61,179	191,164	152,198	403,072
Basic earnings per share	$ 1.45	$ (0.07)		$ 1.04	$ 1.06	$ (0.43)		$ 1.77	$ 0.33	$ 1.12	$ 3.49	$ 2.78	$ 7.36
Diluted earnings per share	$ 1.45	$ (0.07)	[1]	$ 1.04	$ 1.06	$ (0.43)	[1]	$ 1.77	$ 0.32	$ 1.12	$ 3.49	$ 2.78	$ 7.32
Dividends declared per share	$ 0.61	$ 0.60		$ 0.60	$ 0.60	$ 0.60		$ 0.59	$ 0.59	$ 0.59	$ 0.61	$ 0.60
Parent Company [Member]
Income (loss) before income taxes											(4,063)	(12,754)	2,690
Net income											$ 191,164	$ 152,198	$ 403,072

[1] The dilutive impact of incremental shares is excluded from loss position in accordance with GAAP.