Segment Reporting (Schedule Of Underwriting Results For Operating Segments To Income (Loss) Before Taxes) (Details) - USD ($) $ in Thousands	3 Months Ended					6 Months Ended		9 Months Ended	12 Months Ended
	Jun. 30, 2017	Mar. 31, 2017	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Jun. 30, 2017	Jun. 30, 2016	Sep. 30, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Segment Reporting [Abstract]
Underwriting gain (loss)	$ 46,259			$ 21,580		$ 115,190	$ 81,851
Net investment income	71,900			73,872		132,749	132,317
Net realized capital gains (losses)	(92,291)			29,165		25,477	(37,212)
Corporate expense	(1,512)			(2,010)		(5,109)	(4,346)
Interest, fee and bond issue cost amortization expense	(7,954)			(8,858)		(16,813)	(17,717)
Other income (expense)	10,655			(10,700)		20,510	2,402
INCOME (LOSS) BEFORE TAXES	$ 27,057	$ 244,947	$ 98,728	$ 103,049	$ 54,246	$ 272,004	$ 157,295	$ 256,023	$ 398,991	$ 607,286	$ 658,115