Fair Value (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018	Sep. 30, 2018
Fixed income securities [Abstract]
Total fixed income securities	$ 757,841	$ 592,645
Equity securities [Abstract]
Total equity securities	72,837	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	14,426	10,687
Agency mortgage-backed securities	51,246	37,385
Asset-backed securities	99,295	64,422
Bank loans	14,625	9,750
Certificates of deposit	2,835	2,835
Collateralized mortgage obligations	5,718	5,423
Corporate securities	262,656	186,651
Options embedded in convertible securities	4,567	3,799
Mortgage-backed securities	47,381	38,540
Municipal obligations	32,986	29,155
Non-U.S. government obligations	24,068	25,180
U.S. government obligations	198,038	178,818
Total fixed income securities	757,841	592,645
Equity securities [Abstract]
Consumer	15,387	17,945
Energy	3,010	3,179
Financial	29,829	25,253
Industrial	4,488	6,920
Technology	2,609	2,303
Funds (e.g. mutual funds, closed end funds, ETFs)	9,438	5,489
Other	8,076	5,333
Total equity securities	72,837	66,422
Short-term investments	1,000	1,000
Cash equivalents	78,072	156,855
Total	909,750	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	15,387	17,945
Energy	3,010	3,179
Financial	29,829	25,253
Industrial	4,488	6,920
Technology	2,609	2,303
Funds (e.g. mutual funds, closed end funds, ETFs)	9,438	5,489
Other	8,076	5,333
Total equity securities	72,837	66,422
Short-term investments	1,000	1,000
Cash equivalents	0	0
Total	76,672	70,257
Recurring [Member] | Level 2 [Member]
Fixed income securities [Abstract]
Agency collateralized mortgage obligations	14,426	10,687
Agency mortgage-backed securities	51,246	37,385
Asset-backed securities	99,295	64,422
Bank loans	14,625	9,750
Certificates of deposit	0	0
Collateralized mortgage obligations	5,718	5,423
Corporate securities	262,656	186,651
Options embedded in convertible securities	4,567	3,799
Mortgage-backed securities	47,381	38,540
Municipal obligations	32,986	29,155
Non-U.S. government obligations	24,068	25,180
U.S. government obligations	198,038	178,818
Total fixed income securities	755,006	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 investments	0	0
Cash equivalents	78,072	156,855
Total	833,078	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 investments	0	0
Cash equivalents	0	0
Total	$ 0	$ 0