Other Comprehensive Income (Loss) (Reclassification From Accumulated Other Comprehensive Income) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income tax expense (benefit)	$ (29,451)	$ 11,378	$ (15,404)	$ (88,092)
NET INCOME (LOSS)	243,057	104,398	450,549	791,817
URA(D) On Securities [Member] | Reclassification From Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other net realized capital gains (losses)	(12,678)	(74)	18,650	(3,597)
Income tax expense (benefit)	1,225	(455)	(5,961)	(623)
NET INCOME (LOSS)	(11,453)	(529)	12,689	(4,220)
Benefit Plan Net Gain (Loss) [Member] | Reclassification From Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other underwriting expenses	2,285	1,726	5,736	4,640
Income tax expense (benefit)	(479)	(363)	(1,204)	(975)
NET INCOME (LOSS)	$ 1,806	$ 1,363	$ 4,532	$ 3,665