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, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
United States corporate tax rate amount	$ (1,650)	$ 30,553	$ 220,472
Change in Tax Status of UK Subsidiaries One-time Tax Expense	103,281	0	0
Nondeductible loss on investments managed by MCIM	26,552	16,231	0
Foreign operations	4,951	37,207	4,672
Tax-exempt investment income amount	(18,927)	(41,565)	(39,710)
TCJA amount	(5,699)	(339,899)	0
Tax credits amount	(3,617)	(10,236)	(13,294)
Stock based compensation amount	(2,635)	(9,001)	(5,411)
Other amount	20,242	3,247	2,748
Income tax expense (benefit)	$ 122,498	$ (313,463)	$ 169,477