Segment Information - Reconciliation of Operating Segments by Individual Segment (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Segment Reporting Information [Line Items]
Total underwriting profit (loss) of insurance segments	$ 16,751	$ (32,802)	$ 38,970	$ (742)
Other operating expenses of the Corporate and Other segment	(7,805)	(7,302)	(23,556)	(22,641)
Underwriting profit (loss)	8,946	(40,104)	15,414	(23,383)
Net investment income	14,959	17,878	51,145	54,844
Net realized and unrealized gains (losses) on investments	8,929	(2,357)	(27,885)	331
Amortization of intangible assets	(149)	(149)	(447)	(447)
Other income and expenses	192	(91)	(967)	(223)
Interest expense	(2,129)	(2,594)	(7,970)	(8,086)
Income (loss) before taxes	30,748	(27,417)	29,290	23,036
Excess and Surplus Lines
Segment Reporting Information [Line Items]
Total underwriting profit (loss) of insurance segments	15,581	(29,351)	39,788	(439)
Net investment income	3,751	3,467	15,028	13,240
Interest expense	0	0	0	0
Specialty Admitted Insurance
Segment Reporting Information [Line Items]
Total underwriting profit (loss) of insurance segments	1,859	837	2,301	3,758
Net investment income	807	976	2,547	2,791
Interest expense	0	0	0	0
Casualty Reinsurance
Segment Reporting Information [Line Items]
Total underwriting profit (loss) of insurance segments	(689)	(4,288)	(3,119)	(4,061)
Net investment income	10,033	11,717	32,171	34,875
Interest expense	$ 0	$ 0	$ 0	$ 0