Fair Value (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Fixed income securities [Abstract]
Total fixed income securities	$ 592,645	$ 521,853
Equity securities [Abstract]
Total equity securities	66,422	201,763
Recurring [Member]
Fixed income securities [Abstract]
Agency collateralized mortgage obligations	10,687	16,586
Agency mortgage-backed securities	37,385	27,075
Asset-backed securities	64,422	43,469
Bank loans	9,750	19,488
Certificates of deposit	2,835	3,135
Collateralized mortgage obligations	5,423	6,492
Corporate securities	186,651	193,058
Options embedded in convertible securities	3,799	5,291
Mortgage-backed securities	38,540	24,204
Municipal obligations	29,155	96,650
Non-U.S. government obligations	25,180	37,394
U.S. government obligations	178,818	49,011
Total fixed income securities	592,645	521,853
Equity securities [Abstract]
Consumer	17,945	46,578
Energy	3,179	10,278
Financial	25,253	45,470
Industrial	6,920	25,402
Technology	2,303	13,061
Funds (e.g. mutual funds, closed end funds, ETFs)	5,489	50,291
Other	5,333	10,683
Total equity securities	66,422	201,763
Short-term	1,000	1,000
Cash equivalents	156,855	59,173
Total	816,922	783,789
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	3,135
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	3,135
Equity securities [Abstract]
Consumer	17,945	46,578
Energy	3,179	10,278
Financial	25,253	45,470
Industrial	6,920	25,402
Technology	2,303	13,061
Funds (e.g. mutual funds, closed end funds, ETFs)	5,489	45,276
Other	5,333	10,683
Total equity securities	66,422	196,748
Short-term	1,000	1,000
Cash equivalents	0	0
Total	70,257	200,883
Recurring [Member] | Level 2 [Member]
Fixed income securities [Abstract]
Agency collateralized mortgage obligations	10,687	16,586
Agency mortgage-backed securities	37,385	27,075
Asset-backed securities	64,422	43,469
Bank loans	9,750	19,488
Certificates of deposit	0	0
Collateralized mortgage obligations	5,423	6,492
Corporate securities	186,651	193,058
Options embedded in convertible securities	3,799	5,291
Mortgage-backed securities	38,540	24,204
Municipal obligations	29,155	96,650
Non-U.S. government obligations	25,180	37,394
U.S. government obligations	178,818	49,011
Total fixed income securities	589,810	518,718
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	5,015
Other	0	0
Total equity securities	0	5,015
Short-term	0	0
Cash equivalents	156,855	59,173
Total	746,665	582,906
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