Short-term Debt and Credit Lines - Schedule of Short-Term Debt (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Short-term Debt
Total short-term debt	$ 44,550	$ 37,950
Notes payable to banks
Short-term Debt
Total short-term debt	8,300	8,200
Commercial paper
Short-term Debt
Total short-term debt	$ 36,250	$ 29,750