Business Segments (Financial Information of Company Operating Segments) (Details) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2025 USD ($)	Jun. 30, 2024 USD ($)	Jun. 30, 2025 USD ($) segment	Jun. 30, 2024 USD ($)
Segment Reporting [Abstract]
Number of reportable segments | segment			2
Segment Reporting Information [Line Items]
Earned Premiums (1)	$ 3,098,185	$ 2,846,415	$ 6,110,566	$ 5,610,762
Investment Income (Loss)	379,303	372,129	739,595	691,967
Other	193,320	95,488	368,046	268,076
Total revenues	3,670,808	3,314,032	7,218,207	6,570,805
Net Income (Loss) to Common Stockholders	401,288	371,909	818,860	814,380
Net investment gains	30,973	(58,512)	47,328	(32,733)
Net investment gains, net of tax	24,288	(46,111)	37,115	(26,964)
Income before income taxes	521,863	487,673	1,060,469	1,061,744
Policyholder Benefits and Claims Incurred, Net	1,955,424	1,780,596	3,856,216	3,444,374
Policy Acquisition And Insurance Operating Expenses	882,099	811,997	1,720,345	1,603,529
Segment Reporting, Other Segment Item, Amount	311,422	233,766	581,177	461,158
Total	3,148,945	2,826,359	6,157,738	5,509,061
Net investment gains (losses)	30,973	(58,512)	47,328	(32,733)
Corporate Reconciling Items And Eliminations
Segment Reporting Information [Line Items]
Investment Income (Loss)	(1,031)	10,096	1,583	31,946
Other	152,193	144,409	300,612	281,795
Total revenues	151,162	154,505	302,195	313,741
Net Income (Loss) to Common Stockholders	(116,490)	(53,865)	(200,710)	(97,757)
Income before income taxes	(149,081)	(68,317)	(256,715)	(125,799)
Segment Reporting, Other Segment Item, Amount	300,243	222,822	558,910	439,540
Total	300,243	222,822	558,910	439,540
Segment Reporting, Reconciling Item, Corporate Nonsegment
Segment Reporting Information [Line Items]
Income before income taxes	30,973	(58,512)	47,328	(32,733)
Net investment gains (losses)	30,973
Insurance
Segment Reporting Information [Line Items]
Earned Premiums (1)		2,484,569		4,883,338
Insurance | Operating Segments
Segment Reporting Information [Line Items]
Earned Premiums (1)	2,728,784	2,484,569	5,371,291	4,883,338
Investment Income (Loss)	299,476	303,561	590,724	548,338
Other	10,154	9,591	20,106	19,014
Total revenues	3,038,414	2,797,721	5,982,121	5,450,690
Net Income (Loss) to Common Stockholders	393,450	374,247	786,572	739,337
Income before income taxes	512,672	490,053	1,022,177	968,202
Policyholder Benefits and Claims Incurred, Net	1,742,235	1,589,569	3,429,688	3,071,121
Policy Acquisition And Insurance Operating Expenses	772,328	707,155	1,507,989	1,389,749
Segment Reporting, Other Segment Item, Amount	11,179	10,944	22,267	21,618
Total	2,525,742	2,307,668	4,959,944	4,482,488
Insurance | Operating Segments | Non-US
Segment Reporting Information [Line Items]
Total revenues	361,000	368,000	694,000	762,000
Reinsurance and Monoline Excess Segment
Segment Reporting Information [Line Items]
Earned Premiums (1)		361,846		727,424
Reinsurance and Monoline Excess Segment | Operating Segments
Segment Reporting Information [Line Items]
Earned Premiums (1)	369,401	361,846	739,275	727,424
Investment Income (Loss)	80,858	58,472	147,288	111,683
Other	0	0	0	0
Total revenues	450,259	420,318	886,563	839,107
Net Income (Loss) to Common Stockholders	100,040	97,638	195,883	199,764
Income before income taxes	127,299	124,449	247,679	252,074
Policyholder Benefits and Claims Incurred, Net	213,189	191,027	426,528	373,253
Policy Acquisition And Insurance Operating Expenses	109,771	104,842	212,356	213,780
Total	322,960	295,869	638,884	587,033
Reinsurance and Monoline Excess Segment | Operating Segments | Non-US
Segment Reporting Information [Line Items]
Total revenues	$ 116,000	$ 117,000	$ 246,000	$ 228,000