Investments - Summary of Realized Gains and Losses (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Investments [Abstract]
Fixed maturity securities, gross realized gains	$ 360	$ 18	$ 382	$ 356
Fixed maturity securities, gross realized losses	(252)	(31)	(475)	(327)
Fixed maturity securities	108	(13)	(93)	29
Bank loan participations, gross realized gains	360	471	1,580	1,607
Bank loan participations, gross realized losses	(1,006)	(150)	(1,106)	(689)
Bank loan participations	(646)	321	474	918
Equity securities, gross realized gain	0	0	0	409
Equity securities, gross realized losses	(47)	0	(62)	0
Equity securities, changes in fair values of equity securities	521	0	(1,189)	0
Equity securities	474	0	(1,251)	409
Short-term investments and other, gross realized gains	0	0	0	0
Short-term investments and other, gross realized losses	0	(1)	(4)	(2)
Short-term investments and other	0	(1)	(4)	(2)
Total	$ (64)	$ 307	$ (874)	$ 1,354