Segment Information (Details) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2019 USD ($) segment	Dec. 31, 2018 USD ($)	Dec. 31, 2017 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	$ 249,945	$ 227,196	$ 235,032	$ 255,838	$ 221,841	$ 201,296	$ 222,555	$ 249,420	$ 968,011	$ 895,112	$ 836,334
Gross written premiums, percent									100.00%	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									$ 205,303	$ 151,612	$ 124,688
Gross written premiums, percent									21.20%	16.90%	14.90%
General Liability
Summary of the Company’s gross written premium by customer segments within our Specialty Insurance segment
Gross written premiums									$ 335,197	$ 277,948	$ 272,660
Gross written premiums, percent									34.60%	31.10%	32.60%
Workers' Compensation
Summary of the Company’s gross written premium by customer segments within our Specialty Insurance segment
Gross written premiums									$ 179,432	$ 246,302	$ 235,668
Gross written premiums, percent									18.60%	27.50%	28.20%
Commercial Multiple Peril
Summary of the Company’s gross written premium by customer segments within our Specialty Insurance segment
Gross written premiums									$ 82,126	$ 67,351	$ 73,859
Gross written premiums, percent									8.50%	7.50%	8.80%
All Other Lines
Summary of the Company’s gross written premium by customer segments within our Specialty Insurance segment
Gross written premiums									$ 165,953	$ 151,899	$ 129,459
Gross written premiums, percent									17.10%	17.00%	15.50%
Customer Segment
Summary of the Company’s gross written premium by customer segments within our Specialty Insurance segment
Gross written premiums									$ 899,852	$ 770,893	$ 713,349
Gross written premiums, percent									93.00%	86.10%	85.30%
Construction
Summary of the Company’s gross written premium by customer segments within our Specialty Insurance segment
Gross written premiums									$ 117,918	$ 101,946	$ 73,378
Gross written premiums, percent									12.20%	11.40%	8.80%
Consumer Services
Summary of the Company’s gross written premium by customer segments within our Specialty Insurance segment
Gross written premiums									$ 133,682	$ 107,086	$ 94,384
Gross written premiums, percent									13.80%	12.00%	11.30%
Marine and Energy
Summary of the Company’s gross written premium by customer segments within our Specialty Insurance segment
Gross written premiums									$ 94,071	$ 82,978	$ 79,238
Gross written premiums, percent									9.70%	9.30%	9.50%
Media and Entertainment
Summary of the Company’s gross written premium by customer segments within our Specialty Insurance segment
Gross written premiums									$ 124,950	$ 119,926	$ 114,442
Gross written premiums, percent									12.90%	13.40%	13.70%
Professional Services
Summary of the Company’s gross written premium by customer segments within our Specialty Insurance segment
Gross written premiums									$ 119,326	$ 110,546	$ 112,575
Gross written premiums, percent									12.40%	12.30%	13.40%
Real Estate
Summary of the Company’s gross written premium by customer segments within our Specialty Insurance segment
Gross written premiums									$ 167,635	$ 132,652	$ 132,029
Gross written premiums, percent									17.30%	14.80%	15.80%
Sports
Summary of the Company’s gross written premium by customer segments within our Specialty Insurance segment
Gross written premiums									$ 30,079	$ 23,590	$ 22,224
Gross written premiums, percent									3.10%	2.60%	2.60%
Transportation
Summary of the Company’s gross written premium by customer segments within our Specialty Insurance segment
Gross written premiums									$ 112,191	$ 92,169	$ 85,079
Gross written premiums, percent									11.60%	10.30%	10.20%
Other
Summary of the Company’s gross written premium by customer segments within our Specialty Insurance segment
Gross written premiums									$ 68,159	$ 124,219	$ 122,985
Gross written premiums, percent									7.00%	13.90%	14.70%