Unaudited Quarterly Financial Data (Summary Of Quarterly Financial Data) (Details) - USD ($) $ 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
Unaudited Quarterly Financial Data [Abstract]
Gross written premiums	$ 1,790,142	$ 1,890,002	$ 1,688,038	$ 1,684,882	$ 1,715,970	$ 1,759,190	$ 1,632,284	$ 1,466,231
Net written premiums	1,492,886	1,569,954	1,319,865	1,392,243	1,356,000	1,378,481	1,190,354	1,107,027	$ 5,774,949	$ 5,031,862	$ 2,723,844
Premiums earned	1,414,558	1,428,400	1,375,623	1,270,454	1,312,441	1,230,771	1,179,836	1,116,010	5,489,035	4,839,058	1,949,595
Net investment income	85,376	95,592	90,709	84,534	82,104	90,298	72,070	69,909	356,211	314,381	286,259
Net realized capital gains (losses)	29,206	112,542	142,563	135,056	(112,902)	30,018	(42,271)	(60,201)	419,367	(185,356)	161,553
Total claims and underwriting expenses	1,505,892	1,562,452	1,255,551	1,174,175	2,282,035	1,347,593	1,600,124	1,057,972	5,498,070	6,287,724	2,544,139
Net income (loss)	$ 36,060	$ 61,137	$ 280,922	$ 251,608	$ (705,062)	$ 21,966	$ (267,295)	$ (11,793)	$ 629,727	$ (962,184)	$ 79,870