Financial Instruments with Off Balance Sheet Risk - Narrative (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
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
Collateralized loans	242,000	260,600
Outstanding letters of credit	$ 20,000	$ 0