Short-term Borrowings - Short-term Borrowings (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Short-term Borrowings
Period end balance for short-term borrowings	$ 0	$ 0	$ 4,391
Interest rate at period end (as a percent)	0.00%	0.00%	0.94%
Average daily balances of short-term borrowings	$ 3,288	$ 5,267	$ 7,051
Average daily interest rate (as a percent)	2.35%	1.10%	0.73%
Federal funds purchased
Short-term Borrowings
Average daily balances of short-term borrowings	$ 156	$ 142	$ 156
Average daily interest rate (as a percent)	1.91%	1.30%	0.71%
Securities sold under repurchase agreements
Short-term Borrowings
Period end balance for short-term borrowings	$ 0	$ 0	$ 2,159
Interest rate at period end (as a percent)	0.00%	0.00%	0.10%
Average daily balances of short-term borrowings	$ 1,977	$ 3,730	$ 5,235
Average daily interest rate (as a percent)	2.22%	0.78%	0.41%
Maximum month-end balances of short-term borrowings	$ 0	$ 2,868	$ 3,391
Line of Credit - TCF Commercial Finance Canada, Inc.
Short-term Borrowings
Period end balance for short-term borrowings	$ 0	$ 0	$ 2,232
Interest rate at period end (as a percent)	0.00%	0.00%	1.75%
Average daily balances of short-term borrowings	$ 1,155	$ 1,395	$ 1,660
Average daily interest rate (as a percent)	2.64%	1.92%	1.75%
Maximum month-end balances of short-term borrowings	$ 8,565	$ 6,171	$ 5,907