Schedule VI - Supplementary Information Concerning Property/Casualty Insurance Operations (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters [Abstract]
Deferred Acquisition Costs	$ 153,505	$ 121,101	$ 64,984
Reserves for Unpaid Losses and Loss Adjustment Expenses	10,429,238	9,409,504	7,398,088
Unearned Premiums	746,775	842,618	583,197
Net Premiums Earned	1,153,589	891,708	608,312
Net Investment Income	320,780	269,093	198,602
Net Losses and Loss Expenses Incurred - Current Year	823,658	689,782	437,853
Net Loses and Loss Expenses Incurred - Prior Year	48,917	(235,757)	(244,302)
Net Paid Losses and Loss Expenses	(1,869,268)	(1,384,545)	(945,194)
Amortization of Deferred Acquisition Costs	305,774	192,790	96,028
Net Premiums Written	$ 899,687	$ 978,037	$ 605,597