Fair Value of Financial Instruments - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Percentage of investment portfolio fair value pricing services or index price	92.30%	90.70%
Net investment gains (losses)	$ 7,931	$ (13,410)	$ (5,076)
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Percentage of investment portfolio fair value pricing services or index price	3.00%
Realized loss on impairment of securities	$ 484	1,833
Net investment gains (losses)	$ (7,518)	$ 12,942