Schedule III. Supplementary Insurance Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Deferred Policy Acquisition Costs	$ 1,171,738	$ 849,160	$ 633,521
Future Policy Benefits, Losses, Claims and Loss Expenses	15,892,573	13,294,630	10,381,138
Unearned Premiums	4,559,107	3,531,213	2,763,599
Premium Revenue	6,333,989	5,194,181	3,952,462
Net Investment Income	559,932	319,479	354,038
Benefits, Claims, Losses and Settlement Expenses	4,338,840	3,876,087	2,924,609
Amortization of Deferred Policy Acquisition Costs	1,568,606	1,214,858	897,677
Other Operating Expenses	276,691	212,184	206,687
Net Written Premiums	7,196,160	5,939,375	4,096,333
Other
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Deferred Policy Acquisition Costs	0	0	0
Future Policy Benefits, Losses, Claims and Loss Expenses	0	0	0
Unearned Premiums	0	0	0
Premium Revenue	0	0	0
Net Investment Income	559,932	319,479	354,038
Benefits, Claims, Losses and Settlement Expenses	0	0	0
Amortization of Deferred Policy Acquisition Costs	0	0	0
Other Operating Expenses	0	0	0
Net Written Premiums	0	0	0
Property | Operating Segments
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Deferred Policy Acquisition Costs	202,999	195,423	118,327
Future Policy Benefits, Losses, Claims and Loss Expenses	7,535,832	6,377,688	4,372,453
Unearned Premiums	1,002,149	931,938	659,236
Premium Revenue	2,770,227	2,608,298	1,936,215
Net Investment Income	0	0	0
Benefits, Claims, Losses and Settlement Expenses	2,044,771	2,163,016	1,435,947
Amortization of Deferred Policy Acquisition Costs	547,210	487,178	353,700
Other Operating Expenses	194,355	143,608	135,547
Net Written Premiums	2,847,659	2,868,002	2,037,200
Casualty and Specialty | Operating Segments
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Deferred Policy Acquisition Costs	968,739	653,737	515,194
Future Policy Benefits, Losses, Claims and Loss Expenses	8,356,741	6,916,942	6,008,685
Unearned Premiums	3,556,958	2,599,275	2,104,363
Premium Revenue	3,563,762	2,585,883	2,016,247
Net Investment Income	0		0
Benefits, Claims, Losses and Settlement Expenses	2,294,069	1,713,071	1,488,662
Amortization of Deferred Policy Acquisition Costs	1,021,396	727,680	543,977
Other Operating Expenses	82,336	68,576	71,140
Net Written Premiums	$ 4,348,501	$ 3,071,373	$ 2,059,133