Borrowings - Short-term Borrowings (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Borrowings
Balance outstanding at year end	$ 20,621	$ 12,363
Maximum balance at any month end during the year	21,032	13,195
Average balance for the year	$ 18,416	$ 12,079
Weighted average rate for the year (as a percent)	1.32%	0.43%
Weighted average rate on borrowings at year end	1.29%	0.44%
Estimated fair value at year end	$ 20,621	$ 12,363