Income Taxes (Reconciliations Of United States Corporate Income Tax Rate To Effective Tax Rate On Income Before Income Taxes) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Income taxes at U.S. corporate tax rate	$ 480,020	$ (1,650)	$ 30,553
Foreign operations	14,718	4,951	37,207
Tax-exempt investment income	(18,430)	(18,927)	(41,565)
Change in tax status of UK subsidiaries	(6,658)	103,281	0
Tax credits	(4,104)	(3,617)	(10,236)
Nondeductible loss on investments managed by MCIM	0	26,552	16,231
TCJA	0	(5,699)	(339,899)
Other	20,800	17,607	(5,754)
Income tax expense (benefit)	$ 486,346	$ 122,498	$ (313,463)