Fair Value (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Fixed income securities [Abstract]
Total fixed income securities	$ 851,937	$ 795,538
Equity securities [Abstract]
Total equity securities	48,417	76,812
Recurring [Member]
Fixed income securities [Abstract]
Agency collateralized mortgage obligations	10,106	12,093
Agency mortgage-backed securities	98,651	56,280
Asset-backed securities	102,193	106,397
Bank loans	8,580	14,568
Certificates of deposit	520	2,835
Collateralized mortgage obligations	6,034	5,616
Corporate securities	318,874	276,087
Options embedded in convertible securities	5,242	5,294
Mortgage-backed securities	43,233	47,463
Municipal obligations	43,920	36,286
Non-U.S. government obligations	29,718	24,179
U.S. government obligations	184,866	208,440
Total fixed income securities	851,937	795,538
Equity securities [Abstract]
Consumer	11,220	16,707
Energy	1,059	3,074
Financial	22,357	31,577
Industrial	4,072	4,927
Technology	2,481	2,817
Funds (e.g., mutual funds, closed end funds, ETFs)	0	9,460
Other	7,228	8,250
Total equity securities	48,417	76,812
Short-term investments	1,000	1,000
Cash equivalents	78,401	59,780
Total	979,755	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	520	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	520	2,835
Equity securities [Abstract]
Consumer	11,220	16,707
Energy	1,059	3,074
Financial	22,357	31,577
Industrial	4,072	4,927
Technology	2,481	2,817
Funds (e.g., mutual funds, closed end funds, ETFs)	0	9,460
Other	7,228	8,250
Total equity securities	48,417	76,812
Short-term investments	1,000	1,000
Cash equivalents	0	0
Total	49,937	80,647
Recurring [Member] | Level 2 [Member]
Fixed income securities [Abstract]
Agency collateralized mortgage obligations	10,106	12,093
Agency mortgage-backed securities	98,651	56,280
Asset-backed securities	102,193	106,397
Bank loans	8,580	14,568
Certificates of deposit	0	0
Collateralized mortgage obligations	6,034	5,616
Corporate securities	318,874	276,087
Options embedded in convertible securities	5,242	5,294
Mortgage-backed securities	43,233	47,463
Municipal obligations	43,920	36,286
Non-U.S. government obligations	29,718	24,179
U.S. government obligations	184,866	208,440
Total fixed income securities	851,417	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	78,401	59,780
Total	929,818	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