Investments - Schedule of Realized Gain (Loss) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Investments, Debt and Equity Securities [Abstract]
Gross gains realized	$ 22	$ 86	$ 63
Gross losses realized	0	0	(40)
Proceeds from sales	$ 144,076	$ 2,493	$ 41,069