Financial Instruments with Off Balance Sheet Risk - Narrative (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Letter of credit with FHLB
Financial Instruments with Off Balance Sheet Risk
Outstanding letters of credit	$ 150	$ 150
Letter of credit with Bank of North Dakota
Financial Instruments with Off Balance Sheet Risk
Outstanding letters of credit	0	20,000
Collateralized loans	$ 245,700	$ 242,000