Schedule III - Supplementary Insurance Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Supplementary Insurance Information, by Segment [Line Items]
Deferred Acquisition Costs	$ 872,289	$ 622,053	$ 581,863
Reverse for Losses and Loss Adjustment Expenses	19,009,486	16,398,997	13,611,313
Unearned Premium Reserves	4,609,634	3,501,359	3,056,735
Premiums Earned	10,406,441	8,681,513	7,403,686
Net Investment Income	1,164,892	642,465	647,139
Incurred Loss and Loss Adjustment Expenses	7,391,253	6,550,837	4,922,898
Amortization of Deferred Acquisition Costs	2,208,766	1,873,250	1,703,726
Other Operating Expenses	582,647	511,237	440,899
Net Written Premium	11,445,505	9,116,983	7,824,424
Reinsurance [Member]
Supplementary Insurance Information, by Segment [Line Items]
Deferred Acquisition Costs	653,929	447,632	420,182
Reverse for Losses and Loss Adjustment Expenses	13,894,992	12,023,446	10,064,970
Unearned Premium Reserves	2,722,771	1,995,218	1,740,357
Premiums Earned	7,757,483	6,466,106	5,491,296
Net Investment Income	823,163	458,354	458,492
Incurred Loss and Loss Adjustment Expenses	5,556,444	4,933,411	3,675,178
Amortization of Deferred Acquisition Costs	1,854,466	1,552,371	1,400,247
Other Operating Expenses	199,148	175,734	160,834
Net Written Premium	8,535,618	6,767,579	5,732,272
Insurance [Member]
Supplementary Insurance Information, by Segment [Line Items]
Deferred Acquisition Costs	218,360	174,421	161,681
Reverse for Losses and Loss Adjustment Expenses	5,114,494	4,375,551	3,546,343
Unearned Premium Reserves	1,886,864	1,506,141	1,316,378
Premiums Earned	2,648,957	2,215,407	1,912,390
Net Investment Income	341,729	184,111	188,647
Incurred Loss and Loss Adjustment Expenses	1,834,809	1,617,426	1,247,720
Amortization of Deferred Acquisition Costs	354,300	320,879	303,479
Other Operating Expenses	383,499	335,503	280,065
Net Written Premium	$ 2,909,886	$ 2,349,404	$ 2,092,152