Investments - Summary of Net Investment Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Net Investment Income [Line Items]
Gross investment income	$ 80,373	$ 65,490	$ 65,049
Investment expense	(4,721)	(4,234)	(3,930)
Net investment income	75,652	61,256	61,119
Fixed maturity securities:
Net Investment Income [Line Items]
Gross investment income	39,875	34,129	26,833
Bank loan participations
Net Investment Income [Line Items]
Gross investment income	19,772	18,279	17,388
Equity securities
Net Investment Income [Line Items]
Gross investment income	5,262	5,240	5,045
Other invested assets
Net Investment Income [Line Items]
Gross investment income	6,254	5,165	14,079
Cash, cash equivalents, restricted cash equivalents, and short-term investments
Net Investment Income [Line Items]
Gross investment income	$ 9,210	$ 2,677	$ 1,704