Fair Value (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Fixed maturities [Abstract]
Total fixed maturities	$ 590,961	$ 521,853
Equity securities [Abstract]
Total equity securities	109,099	201,763
Recurring [Member]
Fixed maturities [Abstract]
Agency collateralized mortgage obligations	10,471	16,586
Agency mortgage-backed securities	36,181	27,075
Asset-backed securities	53,881	43,469
Bank loans	17,067	19,488
Certificates of deposit	3,126	3,135
Collateralized mortgage obligations	5,131	6,492
Corporate securities	193,325	193,058
Options embedded in convertible securities	5,373	5,291
Mortgage-backed securities	32,790	24,204
Municipal obligations	30,594	96,650
Non-U.S. government obligations	39,750	37,394
U.S. government obligations	163,272	49,011
Total fixed maturities	590,961	521,853
Equity securities [Abstract]
Consumer	25,632	46,578
Energy	6,438	10,278
Financial	39,917	45,470
Industrial	12,011	25,402
Technology	3,657	13,061
Funds (e.g. mutual funds, closed end funds, ETFs)	9,587	50,291
Other	11,857	10,683
Total equity securities	109,099	201,763
Short-term	1,000	1,000
Cash equivalents	104,662	59,173
Total fair value	805,722	783,789
Recurring [Member] | Level 1 [Member]
Fixed maturities [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	3,126	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 maturities	3,126	3,135
Equity securities [Abstract]
Consumer	25,632	46,578
Energy	6,438	10,278
Financial	39,917	45,470
Industrial	12,011	25,402
Technology	3,657	13,061
Funds (e.g. mutual funds, closed end funds, ETFs)	9,587	45,276
Other	11,857	10,683
Total equity securities	109,099	196,748
Short-term	1,000	1,000
Cash equivalents	0	0
Total fair value	113,225	200,883
Recurring [Member] | Level 2 [Member]
Fixed maturities [Abstract]
Agency collateralized mortgage obligations	10,471	16,586
Agency mortgage-backed securities	36,181	27,075
Asset-backed securities	53,881	43,469
Bank loans	17,067	19,488
Certificates of deposit	0	0
Collateralized mortgage obligations	5,131	6,492
Corporate securities	193,325	193,058
Options embedded in convertible securities	5,373	5,291
Mortgage-backed securities	29,162	24,204
Municipal obligations	30,594	96,650
Non-U.S. government obligations	39,750	37,394
U.S. government obligations	163,272	49,011
Total fixed maturities	584,207	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	104,662	59,173
Total fair value	688,869	582,906
Recurring [Member] | Level 3 [Member]
Fixed maturities [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	3,628	0
Municipal obligations	0	0
Non-U.S. government obligations	0	0
U.S. government obligations	0	0
Total fixed maturities	3,628	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 fair value	$ 3,628	$ 0