Borrowed Funds - Short-term Borrowings and Repurchase Agreements (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Short-term Borrowings from FHLB
Short-term Borrowings and Repurchase Agreements
Balance at end of period	$ 81,810	$ 87,733
Average balance during the period	121,199	137,822
Maximum month-end balance	$ 220,097	$ 210,600
Weighted-average rate during the year	1.25%	0.51%
Weighted-average rate at end of period	1.27%	0.74%
Investment Securities
Short-term Borrowings and Repurchase Agreements
Investment securities held as collateral	$ 25,800	$ 26,000
Repurchase Agreements
Short-term Borrowings and Repurchase Agreements
Balance at end of period	25,045	25,160
Average balance during the period	25,096	27,066
Maximum month-end balance	$ 25,814	$ 29,561
Weighted-average rate during the year	0.30%	0.27%
Weighted-average rate at end of period	0.30%	0.28%