Fair Value of Financial Instruments - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Percentage of investment portfolio fair value pricing services or index price	90.70%	90.70%
Net realized investment gains (losses)	$ (13,410)	$ (5,076)	$ 2,529
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	2.80%
Realized loss on impairment of securities	$ 1,833	1,889
Net realized investment gains (losses)	$ 12,942	$ 5,011