Fair Value, Assets Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Fixed income securities [Abstract]
Total, Fair Value	$ 919,692	$ 795,538
Equity securities [Abstract]
Total equity securities	58,169	76,812
Transfers Between Levels [Abstract]
Transfers of assets from Level 1 to Level 2	0	0
Transfers of assets from Level 2 to Level 1	0	0
Recurring [Member]
Fixed income securities [Abstract]
Agency collateralized mortgage obligations	11,931	12,093
Agency mortgage-backed securities	102,107	56,280
Asset-backed securities	107,696	106,397
Bank loans	11,361	14,568
Certificates of deposit		2,835
Collateralized mortgage obligations	5,118	5,616
Corporate securities	352,837	276,087
Options embedded in convertible securities	7,404	5,294
Mortgage-backed securities	38,056	47,463
Municipal obligations	45,143	36,286
Non-U.S. government obligations	30,600	24,179
U.S. government obligations	207,439	208,440
Total, Fair Value	919,692	795,538
Equity securities [Abstract]
Consumer	11,598	16,707
Energy	1,227	3,074
Financial	29,064	31,577
Industrial	5,180	4,927
Technology	2,851	2,817
Funds (e.g., mutual funds, closed end funds, ETFs)		9,460
Other	8,249	8,250
Total equity securities	58,169	76,812
Short-term investments	1,000	1,000
Cash equivalents	47,026	59,780
Total	1,025,887	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
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, Fair Value	0	2,835
Equity securities [Abstract]
Consumer	11,598	16,707
Energy	1,227	3,074
Financial	29,064	31,577
Industrial	5,180	4,927
Technology	2,851	2,817
Funds (e.g., mutual funds, closed end funds, ETFs)		9,460
Other	8,249	8,250
Total equity securities	58,169	76,812
Short-term investments	1,000	1,000
Cash equivalents	0	0
Total	59,169	80,647
Recurring [Member] | Level 2 [Member]
Fixed income securities [Abstract]
Agency collateralized mortgage obligations	11,931	12,093
Agency mortgage-backed securities	102,107	56,280
Asset-backed securities	107,696	106,397
Bank loans	11,361	14,568
Certificates of deposit		0
Collateralized mortgage obligations	5,118	5,616
Corporate securities	352,837	276,087
Options embedded in convertible securities	7,404	5,294
Mortgage-backed securities	38,056	47,463
Municipal obligations	45,143	36,286
Non-U.S. government obligations	30,600	24,179
U.S. government obligations	207,439	208,440
Total, Fair Value	919,692	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
Other	0	0
Total equity securities	0	0
Short-term investments	0	0
Cash equivalents	47,026	59,780
Total	966,718	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
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, Fair Value	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
Other	0	0
Total equity securities	0	0
Short-term investments	0	0
Cash equivalents	0	0
Total	$ 0	$ 0