Segment Information - Schedule of Revenue by Segment (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Segment Reporting Information [Line Items]
Net premiums earned:	$ 1,080,231	$ 942,150	$ 2,131,175	$ 1,844,486
Miscellaneous income	5,835	6,104	13,636	8,738
Net investment income earned	108,642	97,696	216,491	189,202
Net realized and unrealized investment gains (losses)	1,297	(5,426)	(338)	(2,082)
Total Investments revenue	109,939	92,270	216,153	187,120
Total revenues	1,196,005	1,040,524	2,360,964	2,040,344
Standard Commercial Lines
Segment Reporting Information [Line Items]
Total revenues	858,707	768,277	1,699,951	1,502,073
Standard Commercial Lines | General liability
Segment Reporting Information [Line Items]
Net premiums earned:	280,097	254,510	553,512	497,859
Standard Commercial Lines | Commercial automobile
Segment Reporting Information [Line Items]
Net premiums earned:	260,652	225,067	512,372	442,438
Standard Commercial Lines | Commercial property
Segment Reporting Information [Line Items]
Net premiums earned:	168,511	141,348	330,064	276,640
Standard Commercial Lines | Workers compensation
Segment Reporting Information [Line Items]
Net premiums earned:	82,316	88,746	170,093	172,930
Standard Commercial Lines | Businessowners' policies
Segment Reporting Information [Line Items]
Net premiums earned:	41,641	34,385	81,562	67,556
Standard Commercial Lines | Bonds
Segment Reporting Information [Line Items]
Net premiums earned:	12,468	11,619	24,556	23,016
Standard Commercial Lines | Other
Segment Reporting Information [Line Items]
Net premiums earned:	7,808	7,034	15,444	13,885
Standard Commercial Lines | Miscellaneous income
Segment Reporting Information [Line Items]
Miscellaneous income	5,214	5,568	12,348	7,749
Standard Personal Lines
Segment Reporting Information [Line Items]
Total revenues	107,015	87,706	211,502	170,029
Standard Personal Lines | Miscellaneous income
Segment Reporting Information [Line Items]
Miscellaneous income	594	536	1,235	989
Standard Personal Lines | Personal automobile
Segment Reporting Information [Line Items]
Net premiums earned:	57,544	48,230	114,504	93,144
Standard Personal Lines | Homeowners
Segment Reporting Information [Line Items]
Net premiums earned:	46,055	36,902	90,168	71,915
Standard Personal Lines | Other
Segment Reporting Information [Line Items]
Net premiums earned:	2,822	2,038	5,595	3,981
E&S Lines
Segment Reporting Information [Line Items]
Total revenues	120,344	92,271	233,358	181,122
E&S Lines | Miscellaneous income
Segment Reporting Information [Line Items]
Miscellaneous income	27	0	53	0
E&S Lines | Casualty lines
Segment Reporting Information [Line Items]
Net premiums earned:	73,887	62,151	145,525	122,968
E&S Lines | Property lines
Segment Reporting Information [Line Items]
Net premiums earned:	$ 46,430	$ 30,120	$ 87,780	$ 58,154