Fair Value (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018
Fixed income securities [Abstract]
Total fixed income securities	$ 737,148	$ 592,645
Equity securities [Abstract]
Total equity securities	74,555	66,422
Transfers Between Levels [Abstract]
Transfers of assets from Level 1 to Level 2	0		$ 0
Transfers of assets from Level 2 to Level 1	0		$ 0
Recurring [Member]
Fixed income securities [Abstract]
Agency collateralized mortgage obligations	15,730	10,687
Agency mortgage-backed securities	41,031	37,385
Asset-backed securities	94,880	64,422
Bank loans	16,074	9,750
Certificates of deposit	2,835	2,835
Collateralized mortgage obligations	6,165	5,423
Corporate securities	251,394	186,651
Options embedded in convertible securities	4,924	3,799
Mortgage-backed securities	42,506	38,540
Municipal obligations	32,834	29,155
Non-U.S. government obligations	31,378	25,180
U.S. government obligations	197,397	178,818
Total fixed income securities	737,148	592,645
Equity securities [Abstract]
Consumer	17,125	17,945
Energy	3,340	3,179
Financial	29,117	25,253
Industrial	4,176	6,920
Technology	2,414	2,303
Funds (e.g. mutual funds, closed end funds, ETFs)	9,796	5,489
Other	8,587	5,333
Total equity securities	74,555	66,422
Short-term	1,000	1,000
Cash equivalents	76,967	156,855
Total	889,670	816,922
Recurring [Member] | Level 1 [Member]
Fixed income securities [Abstract]
Agency collateralized mortgage obligations	0	0
Agency mortgage-backed securities	0	0
Asset-backed securities	0	0
Bank loans	0	0
Certificates of deposit	2,835	2,835
Collateralized mortgage obligations	0	0
Corporate securities	0	0
Options embedded in convertible securities	0	0
Mortgage-backed securities	0	0
Municipal obligations	0	0
Non-U.S. government obligations	0	0
U.S. government obligations	0	0
Total fixed income securities	2,835	2,835
Equity securities [Abstract]
Consumer	17,125	17,945
Energy	3,340	3,179
Financial	29,117	25,253
Industrial	4,176	6,920
Technology	2,414	2,303
Funds (e.g. mutual funds, closed end funds, ETFs)	9,796	5,489
Other	8,587	5,333
Total equity securities	74,555	66,422
Short-term	1,000	1,000
Cash equivalents	0	0
Total	78,390	70,257
Recurring [Member] | Level 2 [Member]
Fixed income securities [Abstract]
Agency collateralized mortgage obligations	15,730	10,687
Agency mortgage-backed securities	41,031	37,385
Asset-backed securities	94,880	64,422
Bank loans	16,074	9,750
Certificates of deposit	0	0
Collateralized mortgage obligations	6,165	5,423
Corporate securities	251,394	186,651
Options embedded in convertible securities	4,924	3,799
Mortgage-backed securities	42,506	38,540
Municipal obligations	32,834	29,155
Non-U.S. government obligations	31,378	25,180
U.S. government obligations	197,397	178,818
Total fixed income securities	734,313	589,810
Equity securities [Abstract]
Consumer	0	0
Energy	0	0
Financial	0	0
Industrial	0	0
Technology	0	0
Funds (e.g. mutual funds, closed end funds, ETFs)	0	0
Other	0	0
Total equity securities	0	0
Short-term	0	0
Cash equivalents	76,967	156,855
Total	811,280	746,665
Recurring [Member] | Level 3 [Member]
Fixed income securities [Abstract]
Agency collateralized mortgage obligations	0	0
Agency mortgage-backed securities	0	0
Asset-backed securities	0	0
Bank loans	0	0
Certificates of deposit	0	0
Collateralized mortgage obligations	0	0
Corporate securities	0	0
Options embedded in convertible securities	0	0
Mortgage-backed securities	0	0
Municipal obligations	0	0
Non-U.S. government obligations	0	0
U.S. government obligations	0	0
Total fixed income securities	0	0
Equity securities [Abstract]
Consumer	0	0
Energy	0	0
Financial	0	0
Industrial	0	0
Technology	0	0
Funds (e.g. mutual funds, closed end funds, ETFs)	0	0
Other	0	0
Total equity securities	0	0
Short-term	0	0
Cash equivalents	0	0
Total	$ 0	$ 0