Borrowings - Federal Funds Purchased and Securities Sold Under Agreements to Repurchase (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Short-term Debt [Line Items]
Maximum amount outstanding at any month end	$ 216,094	$ 236,099	$ 192,786
Weighted average interest rate at end of year	0.71%	0.31%	0.28%
Average amount outstanding	$ 171,686	$ 187,560	$ 168,188
Weighted average interest rate during the year	0.46%	0.26%	0.20%