Short-Term Borrowings and Long-Term Debt (Schedule of Debt) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Long-term debt
Long-term debt outstanding at end of period	$ 55,498	$ 65,559	$ 55,617
Weighted average rate on outstanding long-term debt	2.38%	2.27%	2.47%
Maximum outstanding long-term debt of any month end	$ 65,554	$ 65,593	$ 74,668
Average outstanding long-term debt	$ 58,704	$ 60,503	$ 68,924
Approximate average rate paid on long-term debt	2.24%	2.41%	2.26%
Short-term borrowings
Short-term borrowings outstanding at end of period	$ 87,500	$ 79,000	$ 36,000
Weighted average rate on short-term borrowings	1.34%	0.71%	0.38%
Maximum outstanding short-term borrowings at any month end	$ 109,000	$ 79,000	$ 36,000
Average outstanding short-term borrowings	$ 91,797	$ 39,802	$ 13,463
Approximate average rate paid on short-term borrowings	1.15%	0.49%	0.27%