Historical Loss and LAE Development - Additional Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
(Favorable)/Unfavorable Reserve Development
Unpaid losses and LAE, Ceded	$ 608,086	$ 443,729	$ 384,517	$ 364,999
(Favorable)/unfavorable reserve development	(125,463)	(101,053)	(75,284)
Casualty Segment
(Favorable)/Unfavorable Reserve Development
(Favorable)/unfavorable reserve development	(108,632)	(75,075)	(62,497)
Property segment
(Favorable)/Unfavorable Reserve Development
Unpaid losses and LAE, Ceded	65,098	40,257
(Favorable)/unfavorable reserve development	(10,981)	(13,019)	(4,461)
Property segment | Marine
(Favorable)/Unfavorable Reserve Development
(Favorable)/unfavorable reserve development	(6,600)	(6,500)	(2,400)
Property segment | Commercial Property
(Favorable)/Unfavorable Reserve Development
(Favorable)/unfavorable reserve development	(2,000)	(5,200)
Property segment | Specialty Personal
(Favorable)/Unfavorable Reserve Development
(Favorable)/unfavorable reserve development			(1,100)
Surety Segment
(Favorable)/Unfavorable Reserve Development
Unpaid losses and LAE, Ceded	50,743	4,017
(Favorable)/unfavorable reserve development	(5,850)	(12,959)	(8,326)
Surety Segment | Contract Surety Product
(Favorable)/Unfavorable Reserve Development
(Favorable)/unfavorable reserve development	(4,600)	(5,900)	(4,200)
Surety Segment | Commercial Surety Product
(Favorable)/Unfavorable Reserve Development
(Favorable)/unfavorable reserve development		(3,800)	(5,800)
Surety Segment | Miscellaneous Surety Product
(Favorable)/Unfavorable Reserve Development
(Favorable)/unfavorable reserve development			1,700
Primary Occurrence | Casualty Segment
(Favorable)/Unfavorable Reserve Development
Unpaid losses and LAE, Ceded	43,688	35,202
Primary Occurrence | Casualty Segment | General Liability
(Favorable)/Unfavorable Reserve Development
(Favorable)/unfavorable reserve development	(25,300)	(17,900)	(11,800)
Primary Occurrence | Casualty Segment | P&C Package
(Favorable)/Unfavorable Reserve Development
(Favorable)/unfavorable reserve development	(7,500)		(6,300)
Excess Occurrence | Casualty Segment
(Favorable)/Unfavorable Reserve Development
Unpaid losses and LAE, Ceded	122,092	88,528
Excess Occurrence | Casualty Segment | Commercial Products
(Favorable)/Unfavorable Reserve Development
(Favorable)/unfavorable reserve development	(10,500)	(12,500)	(6,800)
Excess Occurrence | Casualty Segment | Personal Umbrella
(Favorable)/Unfavorable Reserve Development
(Favorable)/unfavorable reserve development	(10,400)		(7,800)
Claims Made | Casualty Segment
(Favorable)/Unfavorable Reserve Development
Unpaid losses and LAE, Ceded	271,259	223,020
Claims Made | Casualty Segment | Miscellaneous Professional Liability
(Favorable)/Unfavorable Reserve Development
(Favorable)/unfavorable reserve development	(14,200)	(7,800)	(10,200)
Claims Made | Casualty Segment | Executive Products
(Favorable)/Unfavorable Reserve Development
(Favorable)/unfavorable reserve development	(5,900)		7,300
Claims Made | Casualty Segment | Medical Professional Liability
(Favorable)/Unfavorable Reserve Development
(Favorable)/unfavorable reserve development		(6,100)	2,300
Transportation | Casualty Segment
(Favorable)/Unfavorable Reserve Development
Unpaid losses and LAE, Ceded	66,283	53,251
(Favorable)/unfavorable reserve development	$ (20,100)	$ (19,100)	(16,600)
ASU 2016-13 | Cumulative Effect, Period of Adoption, Adjustment
(Favorable)/Unfavorable Reserve Development
Unpaid losses and LAE, Ceded			$ 1,345