Borrowed Funds - Short-term Borrowings and Repurchase Agreements (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Short-term Borrowings from FHLB
Short-term Borrowings and Repurchase Agreements
Balance at end of period	$ 204,817	$ 149,596
Average balance during the period	148,752	100,969
Maximum month-end balance	$ 204,816	$ 220,097
Weighted-average rate during the year	1.64%	1.16%
Weighted-average rate at end of period	1.87%	1.61%
Investment Securities
Short-term Borrowings and Repurchase Agreements
Investment securities held as collateral	$ 21,300	$ 23,100
Repurchase Agreements
Short-term Borrowings and Repurchase Agreements
Balance at end of period	20,676	22,403
Average balance during the period	20,605	25,160
Maximum month-end balance	$ 20,676	$ 25,972
Weighted-average rate during the year	0.37%	0.30%
Weighted-average rate at end of period	0.39%	0.34%