Fair Value (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Fixed income securities [Abstract]
Total fixed income securities	$ 812,001	$ 795,538
Equity securities [Abstract]
Total equity securities	46,009	76,812
Recurring [Member]
Fixed income securities [Abstract]
Agency collateralized mortgage obligations	11,054	12,093
Agency mortgage-backed securities	91,512	56,280
Asset-backed securities	99,830	106,397
Bank loans	8,121	14,568
Certificates of deposit	2,835	2,835
Collateralized mortgage obligations	6,749	5,616
Corporate securities	290,889	276,087
Options embedded in convertible securities	5,005	5,294
Mortgage-backed securities	45,502	47,463
Municipal obligations	38,359	36,286
Non-U.S. government obligations	30,210	24,179
U.S. government obligations	181,935	208,440
Total fixed income securities	812,001	795,538
Equity securities [Abstract]
Consumer	10,087	16,707
Energy	1,333	3,074
Financial	22,468	31,577
Industrial	3,483	4,927
Technology	2,443	2,817
Funds (e.g., mutual funds, closed end funds, ETFs)	0	9,460
Other	6,195	8,250
Total equity securities	46,009	76,812
Short-term investments	1,000	1,000
Cash equivalents	94,039	59,780
Total	953,049	933,130
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	10,087	16,707
Energy	1,333	3,074
Financial	22,468	31,577
Industrial	3,483	4,927
Technology	2,443	2,817
Funds (e.g., mutual funds, closed end funds, ETFs)	0	9,460
Other	6,195	8,250
Total equity securities	46,009	76,812
Short-term investments	1,000	1,000
Cash equivalents	0	0
Total	49,844	80,647
Recurring [Member] | Level 2 [Member]
Fixed income securities [Abstract]
Agency collateralized mortgage obligations	11,054	12,093
Agency mortgage-backed securities	91,512	56,280
Asset-backed securities	99,830	106,397
Bank loans	8,121	14,568
Certificates of deposit	0	0
Collateralized mortgage obligations	6,749	5,616
Corporate securities	290,889	276,087
Options embedded in convertible securities	5,005	5,294
Mortgage-backed securities	45,502	47,463
Municipal obligations	38,359	36,286
Non-U.S. government obligations	30,210	24,179
U.S. government obligations	181,935	208,440
Total fixed income securities	809,166	792,703
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	94,039	59,780
Total	903,205	852,483
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