Short-Term Borrowings and Long-Term Debt (Schedule of Debt) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Long-term debt
Long-term debt outstanding at end of period	$ 65,559	$ 55,617	$ 74,672
Weighted average rate on outstanding long-term debt	2.27%	2.47%	2.35%
Maximum outstanding long-term debt of any month end	$ 65,593	$ 74,668	$ 75,471
Average outstanding long-term debt	$ 60,503	$ 68,924	$ 74,714
Approximate average rate paid on long-term debt	2.41%	2.26%	2.39%
Short-term borrowings
Short-term borrowings outstanding at end of period	$ 79,000	$ 36,000	$ 2,000
Weighted average rate on short-term borrowings	0.71%	0.38%	0.31%
Maximum outstanding short-term borrowings at any month end	$ 79,000	$ 36,000	$ 15,000
Average outstanding short-term borrowings	$ 39,802	$ 13,463	$ 4,344
Approximate average rate paid on short-term borrowings	0.49%	0.27%	0.28%