Operating Segment Information - Summary of Revenue by Major Product Type (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Segment Reporting Information [Line Items]
Net premiums earned	$ 1,144,436	$ 980,903	$ 865,747
Casualty Segment
Segment Reporting Information [Line Items]
Net premiums earned	711,832	633,639	569,521
Casualty Segment | Commercial excess and personal umbrella
Segment Reporting Information [Line Items]
Net premiums earned	253,921	219,437	178,214
Casualty Segment | General liability
Segment Reporting Information [Line Items]
Net premiums earned	100,374	90,853	91,653
Casualty Segment | Commercial transportation
Segment Reporting Information [Line Items]
Net premiums earned	96,992	83,352	64,624
Casualty Segment | Professional services
Segment Reporting Information [Line Items]
Net premiums earned	95,187	88,855	85,196
Casualty Segment | Small commercial
Segment Reporting Information [Line Items]
Net premiums earned	67,673	64,660	63,357
Casualty Segment | Executive products
Segment Reporting Information [Line Items]
Net premiums earned	26,606	21,873	26,509
Casualty Segment | Other casualty
Segment Reporting Information [Line Items]
Net premiums earned	71,079	64,609	59,968
Property Segment
Segment Reporting Information [Line Items]
Net premiums earned	307,886	231,837	183,720
Property Segment | Commercial property
Segment Reporting Information [Line Items]
Net premiums earned	163,078	107,941	79,406
Property Segment | Marine
Segment Reporting Information [Line Items]
Net premiums earned	113,208	97,745	81,852
Property Segment | Other property
Segment Reporting Information [Line Items]
Net premiums earned	31,600	26,151	22,462
Surety Segment
Segment Reporting Information [Line Items]
Net premiums earned	124,718	115,427	112,506
Surety Segment | Commercial property
Segment Reporting Information [Line Items]
Net premiums earned	47,652	43,738	42,872
Surety Segment | Miscellaneous
Segment Reporting Information [Line Items]
Net premiums earned	45,826	43,982	42,292
Surety Segment | Contract
Segment Reporting Information [Line Items]
Net premiums earned	$ 31,240	$ 27,707	$ 27,342