Fair Value (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Fixed income securities [Abstract]
Total fixed income securities	$ 671,973	$ 592,645
Equity securities [Abstract]
Total equity securities	68,956	66,422
Recurring [Member]
Fixed income securities [Abstract]
Agency collateralized mortgage obligations	15,773	10,687
Agency mortgage-backed securities	41,283	37,385
Asset-backed securities	77,307	64,422
Bank loans	11,284	9,750
Certificates of deposit	2,835	2,835
Collateralized mortgage obligations	5,433	5,423
Corporate securities	222,853	186,651
Options embedded in convertible securities	4,676	3,799
Mortgage-backed securities	37,984	38,540
Municipal obligations	30,357	29,155
Non-U.S. government obligations	30,879	25,180
U.S. government obligations	191,309	178,818
Total fixed income securities	671,973	592,645
Equity securities [Abstract]
Consumer	15,862	17,945
Energy	3,534	3,179
Financial	24,814	25,253
Industrial	7,412	6,920
Technology	2,631	2,303
Funds (e.g. mutual funds, closed end funds, ETFs)	6,324	5,489
Other	8,379	5,333
Total equity securities	68,956	66,422
Short-term	1,000	1,000
Cash equivalents	113,377	156,855
Total	855,306	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	15,862	17,945
Energy	3,534	3,179
Financial	24,814	25,253
Industrial	7,412	6,920
Technology	2,631	2,303
Funds (e.g. mutual funds, closed end funds, ETFs)	6,324	5,489
Other	8,379	5,333
Total equity securities	68,956	66,422
Short-term	1,000	1,000
Cash equivalents	504	0
Total	73,295	70,257
Recurring [Member] | Level 2 [Member]
Fixed income securities [Abstract]
Agency collateralized mortgage obligations	15,773	10,687
Agency mortgage-backed securities	41,283	37,385
Asset-backed securities	77,307	64,422
Bank loans	11,284	9,750
Certificates of deposit	0	0
Collateralized mortgage obligations	5,433	5,423
Corporate securities	222,853	186,651
Options embedded in convertible securities	4,676	3,799
Mortgage-backed securities	37,984	38,540
Municipal obligations	30,357	29,155
Non-U.S. government obligations	30,879	25,180
U.S. government obligations	191,309	178,818
Total fixed income securities	669,138	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	112,873	156,855
Total	782,011	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