Fair Value (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Fixed income securities [Abstract]
Total fixed income securities	$ 746,856	$ 795,538
Equity securities [Abstract]
Total equity securities	70,320	76,812
Transfers Between Levels [Abstract]
Transfers of assets from Level 1 to Level 2	0
Transfers of assets from Level 2 to Level 1	0
Recurring [Member]
Fixed income securities [Abstract]
Agency collateralized mortgage obligations	10,460	12,093
Agency mortgage-backed securities	58,619	56,280
Asset-backed securities	93,369	106,397
Bank loans	10,187	14,568
Certificates of deposit	2,835	2,835
Collateralized mortgage obligations	4,076	5,616
Corporate securities	259,528	276,087
Options embedded in convertible securities	2,632	5,294
Mortgage-backed securities	47,993	47,463
Municipal obligations	39,132	36,286
Non-U.S. government obligations	28,842	24,179
U.S. government obligations	189,183	208,440
Total fixed income securities	746,856	795,538
Equity securities [Abstract]
Consumer	11,880	16,707
Energy	1,508	3,074
Financial	22,092	31,577
Industrial	3,707	4,927
Technology	2,963	2,817
Funds (e.g., mutual funds, closed end funds, ETFs)	21,837	9,460
Other	6,333	8,250
Total equity securities	70,320	76,812
Short-term investments	1,000	1,000
Cash equivalents	69,527	59,780
Total	887,703	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	11,880	16,707
Energy	1,508	3,074
Financial	22,092	31,577
Industrial	3,707	4,927
Technology	2,963	2,817
Funds (e.g., mutual funds, closed end funds, ETFs)	21,837	9,460
Other	6,333	8,250
Total equity securities	70,320	76,812
Short-term investments	1,000	1,000
Cash equivalents	0	0
Total	74,155	80,647
Recurring [Member] | Level 2 [Member]
Fixed income securities [Abstract]
Agency collateralized mortgage obligations	10,460	12,093
Agency mortgage-backed securities	58,619	56,280
Asset-backed securities	93,369	106,397
Bank loans	10,187	14,568
Certificates of deposit	0	0
Collateralized mortgage obligations	4,076	5,616
Corporate securities	259,528	276,087
Options embedded in convertible securities	2,632	5,294
Mortgage-backed securities	47,993	47,463
Municipal obligations	39,132	36,286
Non-U.S. government obligations	28,842	24,179
U.S. government obligations	189,183	208,440
Total fixed income securities	744,021	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	69,527	59,780
Total	813,548	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