Investments - Summary of Realized Gains and Losses (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Net realized investment gains	$ (810)	$ 1,047
Fixed maturity securities
Schedule of Available-for-sale Securities [Line Items]
Gross realized gains	22	338
Gross realized losses	(223)	(296)
Net realized investment gains	(201)	42
Bank loan participations
Schedule of Available-for-sale Securities [Line Items]
Gross realized gains	1,220	1,136
Gross realized losses	(100)	(539)
Net realized investment gains	1,120	597
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Gross realized gains	0	409
Gross realized losses	(15)	0
Changes in fair values of equity securities	(1,710)	0
Net realized investment gains	(1,725)	409
Short-term investments and other
Schedule of Available-for-sale Securities [Line Items]
Gross realized gains	0	0
Gross realized losses	(4)	(1)
Net realized investment gains	$ (4)	$ (1)