Short-Term Borrowings - Components and Selected Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Short-term borrowings
Short-term borrowings		$ 9,151
Federal funds purchased
Short-term borrowings
Highest month-end balance	$ 6,000		$ 8,000
Average outstanding balance	$ 1,280	$ 610	$ 4,727
Weighted-average interest rate paid (as a percent)	0.97%	0.25%	0.05%
Securities sold under agreements to repurchase
Short-term borrowings
Short-term borrowings		$ 9,151
Weighted-average interest rate (as a percent)		0.54%	0.11%
Highest month-end balance	$ 5,951	$ 235,451	$ 520,740
Average outstanding balance	$ 893	$ 112,937	$ 376,902
Weighted-average interest rate paid (as a percent)	0.55%	0.17%	0.05%