Segment Information (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2020 USD ($) segment	Mar. 31, 2019 USD ($)
Segment information
Number of reportable segments | segment	1
Summary of the Company’s gross written premium by customer segments within our Specialty Insurance segment
Gross written premiums	$ 213,784	$ 255,838
Gross written premiums, percent	100.00%	100.00%
Commercial Auto
Summary of the Company’s gross written premium by customer segments within our Specialty Insurance segment
Gross written premiums	$ 40,331	$ 41,784
Gross written premiums, percent	18.90%	16.30%
General Liability
Summary of the Company’s gross written premium by customer segments within our Specialty Insurance segment
Gross written premiums	$ 78,533	$ 71,295
Gross written premiums, percent	36.70%	27.90%
Workers' Compensation
Summary of the Company’s gross written premium by customer segments within our Specialty Insurance segment
Gross written premiums	$ 28,821	$ 90,062
Gross written premiums, percent	13.50%	35.20%
Commercial Multiple Peril
Summary of the Company’s gross written premium by customer segments within our Specialty Insurance segment
Gross written premiums	$ 16,365	$ 19,634
Gross written premiums, percent	7.60%	7.70%
All Other Lines
Summary of the Company’s gross written premium by customer segments within our Specialty Insurance segment
Gross written premiums	$ 49,734	$ 33,063
Gross written premiums, percent	23.30%	12.90%
Customer Segment
Summary of the Company’s gross written premium by customer segments within our Specialty Insurance segment
Gross written premiums	$ 212,284	$ 197,141
Gross written premiums, percent	99.30%	77.10%
Construction
Summary of the Company’s gross written premium by customer segments within our Specialty Insurance segment
Gross written premiums	$ 24,514	$ 23,248
Gross written premiums, percent	11.50%	9.10%
Consumer Services
Summary of the Company’s gross written premium by customer segments within our Specialty Insurance segment
Gross written premiums	$ 30,568	$ 27,485
Gross written premiums, percent	14.30%	10.70%
Marine and Energy
Summary of the Company’s gross written premium by customer segments within our Specialty Insurance segment
Gross written premiums	$ 32,790	$ 19,812
Gross written premiums, percent	15.30%	7.80%
Media and Entertainment
Summary of the Company’s gross written premium by customer segments within our Specialty Insurance segment
Gross written premiums	$ 30,467	$ 29,692
Gross written premiums, percent	14.30%	11.60%
Professional Services
Summary of the Company’s gross written premium by customer segments within our Specialty Insurance segment
Gross written premiums	$ 29,698	$ 29,562
Gross written premiums, percent	13.90%	11.60%
Real Estate
Summary of the Company’s gross written premium by customer segments within our Specialty Insurance segment
Gross written premiums	$ 33,215	$ 28,735
Gross written premiums, percent	15.50%	11.20%
Sports
Summary of the Company’s gross written premium by customer segments within our Specialty Insurance segment
Gross written premiums	$ 9,565	$ 7,850
Gross written premiums, percent	4.50%	3.10%
Transportation
Summary of the Company’s gross written premium by customer segments within our Specialty Insurance segment
Gross written premiums	$ 21,467	$ 30,757
Gross written premiums, percent	10.00%	12.00%
Other
Summary of the Company’s gross written premium by customer segments within our Specialty Insurance segment
Gross written premiums	$ 1,500	$ 58,697
Gross written premiums, percent	0.70%	22.90%