Segment Information - Reconciliation of Underwriting Profit (Loss) of Insurance Segments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Underwriting profit (loss) of operating segments	$ 17,909	$ 54,909	$ 31,094
Other operating expenses	(27,664)	(26,903)	(25,330)
Underwriting profit	(9,755)	28,006	5,764
Net investment income	75,652	61,256	61,119
Net realized investment (losses) gains	(2,919)	(5,479)	(1,989)
Other income	1,137	505	361
Other expenses	(1,055)	(1,300)	(539)
Interest expense	(10,596)	(11,553)	(8,974)
Amortization of intangible assets	(597)	(597)	(597)
Income before income taxes	51,867	70,838	55,145
Excess and Surplus Lines
Segment Reporting Information [Line Items]
Underwriting profit (loss) of operating segments	19,157	42,834	29,693
Net investment income	21,358	14,456	15,014
Interest expense	0	0	0
Amortization of intangible assets	(362)	(362)	(362)
Specialty Admitted Insurance
Segment Reporting Information [Line Items]
Underwriting profit (loss) of operating segments	5,913	6,972	3,166
Net investment income	3,802	3,262	2,532
Interest expense	0	0	0
Amortization of intangible assets	(235)	(235)	(235)
Casualty Reinsurance
Segment Reporting Information [Line Items]
Underwriting profit (loss) of operating segments	(7,161)	5,103	(1,765)
Net investment income	46,325	38,838	31,507
Interest expense	0	0	0
Corporate and other segment
Segment Reporting Information [Line Items]
Underwriting profit (loss) of operating segments	0	0	0
Net investment income	4,167	4,700	12,066
Interest expense	$ (10,596)	$ (11,553)	$ (8,974)