Investments - Investment Income (Expense) Comprised Primarily of Interest and Dividends (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Net Investment Income [Line Items]
Total investment income									$ 33,821	$ 27,776	$ 15,655
Less: Investment expenses									(3,078)	(2,960)	(2,195)
Net investment income	$ 7,578	$ 7,613	$ 7,410	$ 8,142	$ 7,603	$ 6,642	$ 5,786	$ 4,785	30,743	24,816	13,460
Debt Securities [Member]
Net Investment Income [Line Items]
Investment income									24,989	18,198	12,375
Equity Securities [Member]
Net Investment Income [Line Items]
Investment income									2,648	2,978	1,799
Short-term investments [Member]
Net Investment Income [Line Items]
Investment income									0	145	22
Cash and Cash Equivalents [Member]
Net Investment Income [Line Items]
Investment income									5,176	5,540	1,043
Other [Member]
Net Investment Income [Line Items]
Investment income									$ 1,008	$ 915	$ 416