Historical Loss and LAE Development - Additional Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
(Favorable)/Unfavorable Reserve Development
Prior accident years	$ (122,579)	$ (125,463)	$ (101,053)
Casualty Segment
(Favorable)/Unfavorable Reserve Development
Prior accident years	(87,225)	(108,632)	(75,075)
Property segment
(Favorable)/Unfavorable Reserve Development
Prior accident years	(24,927)	(10,981)	(13,019)
Property segment | Marine
(Favorable)/Unfavorable Reserve Development
Prior accident years	(17,000)	(6,600)	(6,500)
Property segment | Commercial Property
(Favorable)/Unfavorable Reserve Development
Prior accident years		(2,000)	(5,200)
Property segment | Homeowners Property
(Favorable)/Unfavorable Reserve Development
Prior accident years	(2,300)
Surety Segment
(Favorable)/Unfavorable Reserve Development
Prior accident years	(10,427)	(5,850)	(12,959)
Surety Segment | Contract
(Favorable)/Unfavorable Reserve Development
Prior accident years	(4,900)	(4,600)	(5,900)
Surety Segment | Commercial
(Favorable)/Unfavorable Reserve Development
Prior accident years	(4,200)		(3,800)
Primary Occurrence | Casualty Segment | General liability
(Favorable)/Unfavorable Reserve Development
Prior accident years	(28,300)	(25,300)	(17,900)
Primary Occurrence | Casualty Segment | Small commercial
(Favorable)/Unfavorable Reserve Development
Prior accident years	(4,600)	(7,500)
Excess Occurrence | Casualty Segment | Commercial Products
(Favorable)/Unfavorable Reserve Development
Prior accident years	(6,300)	(10,500)	(12,500)
Excess Occurrence | Casualty Segment | Personal Umbrella
(Favorable)/Unfavorable Reserve Development
Prior accident years	(3,800)	(10,400)
Claims Made | Casualty Segment | Miscellaneous Professional Liability
(Favorable)/Unfavorable Reserve Development
Prior accident years	(18,800)	(14,200)	(7,800)
Claims Made | Casualty Segment | Executive products
(Favorable)/Unfavorable Reserve Development
Prior accident years	(4,100)	(5,900)
Claims Made | Casualty Segment | Medical Professional Liability
(Favorable)/Unfavorable Reserve Development
Prior accident years			(6,100)
Transportation | Casualty Segment
(Favorable)/Unfavorable Reserve Development
Prior accident years	$ (6,400)	$ (20,100)	$ (19,100)