Fair Value (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Fixed income securities [Abstract]
Total fixed income securities	$ 795,538	$ 592,645
Equity securities [Abstract]
Total equity securities	76,812	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	12,093	10,687
Agency mortgage-backed securities	56,280	37,385
Asset-backed securities	106,397	64,422
Bank loans	14,568	9,750
Certificates of deposit	2,835	2,835
Collateralized mortgage obligations	5,616	5,423
Corporate securities	276,087	186,651
Options embedded in convertible securities	5,294	3,799
Mortgage-backed securities	47,463	38,540
Municipal obligations	36,286	29,155
Non-U.S. government obligations	24,179	25,180
U.S. government obligations	208,440	178,818
Total fixed income securities	795,538	592,645
Equity securities [Abstract]
Consumer	16,707	17,945
Energy	3,074	3,179
Financial	31,577	25,253
Industrial	4,927	6,920
Technology	2,817	2,303
Funds (e.g., mutual funds, closed end funds, ETFs)	9,460	5,489
Other	8,250	5,333
Total equity securities	76,812	66,422
Short-term	1,000	1,000
Cash equivalents	59,780	156,855
Total	933,130	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	16,707	17,945
Energy	3,074	3,179
Financial	31,577	25,253
Industrial	4,927	6,920
Technology	2,817	2,303
Funds (e.g., mutual funds, closed end funds, ETFs)	9,460	5,489
Other	8,250	5,333
Total equity securities	76,812	66,422
Short-term	1,000	1,000
Cash equivalents	0	0
Total	80,647	70,257
Recurring [Member] | Level 2 [Member]
Fixed income securities [Abstract]
Agency collateralized mortgage obligations	12,093	10,687
Agency mortgage-backed securities	56,280	37,385
Asset-backed securities	106,397	64,422
Bank loans	14,568	9,750
Certificates of deposit	0	0
Collateralized mortgage obligations	5,616	5,423
Corporate securities	276,087	186,651
Options embedded in convertible securities	5,294	3,799
Mortgage-backed securities	47,463	38,540
Municipal obligations	36,286	29,155
Non-U.S. government obligations	24,179	25,180
U.S. government obligations	208,440	178,818
Total fixed income securities	792,703	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	59,780	156,855
Total	852,483	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