Other Income, Net (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Other Income and Expenses [Abstract]
Interest income	$ 2,407	$ 129	$ 129
Other nonoperating income	0	5	11
Insurance Policy Expense (Income), Net	(48)	(43)	(40)
Other Nonoperating Expense	0	0	(106)
Other income (expense), net	$ 2,359	$ 91	$ (6)