Schedule III. Supplementary Insurance Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Deferred Policy Acquisition Costs	$ 1,751,437	$ 1,171,738	$ 849,160
Future Policy Benefits, Losses, Claims and Loss Expenses	20,486,869	15,892,573	13,294,630
Unearned Premiums	6,136,135	4,559,107	3,531,213
Premium Revenue	7,471,133	6,333,989	5,194,181
Net Investment Income	1,253,110	559,932	319,479
Benefits, Claims, Losses and Settlement Expenses	3,573,509	4,338,840	3,876,087
Amortization of Deferred Policy Acquisition Costs	1,875,034	1,568,606	1,214,858
Other Operating Expenses	375,182	276,691	212,184
Net Written Premiums	7,467,813	7,196,160	5,939,375
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	1,253,110	559,932	319,479
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	274,288	202,999	195,423
Future Policy Benefits, Losses, Claims and Loss Expenses	7,833,620	7,535,832	6,377,688
Unearned Premiums	1,158,294	1,002,149	931,938
Premium Revenue	3,090,792	2,770,227	2,608,298
Net Investment Income	0	0	0
Benefits, Claims, Losses and Settlement Expenses	799,905	2,044,771	2,163,016
Amortization of Deferred Policy Acquisition Costs	600,127	547,210	487,178
Other Operating Expenses	251,433	194,355	143,608
Net Written Premiums	2,967,309	2,847,659	2,868,002
Casualty and Specialty | Operating Segments
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Deferred Policy Acquisition Costs	1,477,149	968,739	653,737
Future Policy Benefits, Losses, Claims and Loss Expenses	12,653,249	8,356,741	6,916,942
Unearned Premiums	4,977,841	3,556,958	2,599,275
Premium Revenue	4,380,341	3,563,762	2,585,883
Net Investment Income	0	0
Benefits, Claims, Losses and Settlement Expenses	2,773,604	2,294,069	1,713,071
Amortization of Deferred Policy Acquisition Costs	1,274,907	1,021,396	727,680
Other Operating Expenses	123,749	82,336	68,576
Net Written Premiums	$ 4,500,504	$ 4,348,501	$ 3,071,373