Investments Net Investment Income by Major Category (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Net Investment Income [Line Items]
Net investment income	$ 17,812	$ 10,679	$ 9,212
Investment expenses	(686)	(587)	(267)
Investment Income, Net	17,126	10,092	8,945
Debt Securities
Net Investment Income [Line Items]
Net investment income	14,942	9,170	8,092
Equity Securities
Net Investment Income [Line Items]
Net investment income	1,277	996	859
Cash, Cash Equivalents And Short-Term Investments [Member]
Net Investment Income [Line Items]
Net investment income	626	141	25
Partnership Interest [Member]
Net Investment Income [Line Items]
Net investment income	937	352	222
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Net Investment Income [Line Items]
Net investment income	$ 30	$ 20	$ 14