Notes Payable and Long-Term Debt (Short-Term Debt) (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Short-term Debt [Line Items]
Short-term notes payable and current portion of long-term debt	$ 800	$ 1,300
Notes Payable to Banks | Unsecured Debt
Short-term Debt [Line Items]
Short-term notes payable and current portion of long-term debt	$ 800	800
Interest rate description	WSJ
Basis spread on variable rate	0.50%
Line of Credit | Unsecured Debt
Short-term Debt [Line Items]
Short-term notes payable and current portion of long-term debt	$ 0	$ 500
Interest rate description		WSJ
Basis spread on variable rate		0.50%