Fair Value Measurements (Summary Of Changes In Level 3 Investments Measured At Fair Value On A Recurring Basis) (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Fair Value Disclosures [Abstract]
Equity securities, beginning of period		$ 183,913	$ 183,523	$ 191,203	$ 0
Purchases		49,000	0	56,250	195,250
Sales		0	0	(26,674)	(25,000)
Total gains (losses) included in net income (loss)		(51,639)	6,881	(39,505)	20,154
Total gains (losses) included in other comprehensive income		0	0	0	0
Transfers into level 3		0	0	0	0
Transfers out of level 3		0	0	0	0
Equity securities, end of period		181,274	190,404	181,274	190,404
Net unrealized gains (losses) included in net income (loss) relating to assets held at September 30, 2017 and 2016	[1]	$ (51,639)	$ 6,881	$ (39,505)	$ 20,154

[1] Included in net realized investment gains (losses) in the consolidated statements of income (loss) and comprehensive income (loss).