| Pay vs Performance Disclosure | 12 Months Ended |
| Dec. 31, 2022 USD ($) | Dec. 31, 2021 USD ($) | Dec. 31, 2020 USD ($) |
| Pay vs Performance Disclosure [Table] | | | |
| Pay vs Performance [Table Text Block] | Value of Initial Fixed Year Summary Compensation Table Total for CEO (1) Compensation Actually Paid to CEO (1)(5) Average Average Compensation Actually Paid to Non-CEO NEOs (5) Total Peer Group Total Stockholder Return (6) Net Income (in thousands) Return On Average Equity 2022 $1,262,648 $1,060,570 $905,793 (2) $764,704 (2) 112.28 98.03 $46,399 20.71 % 2021 1,024,797 1,623,245 765,149 (3) 1,269,978 (3) 131.21 113.59 49,607 20.33 2020 967,524 725,866 731,858 (4) 490,200 (4) 78.82 85.98 32,712 14.49 | | |
| Company Selected Measure Name | Return On Average Equity | | |
| Named Executive Officers, Footnote [Text Block] | (1) The CEO in 2022, 2021 and 2020 was Mr. Nelson. (2) Non-CEO NEOs in 2022 were Messrs. Olafson, Winterbottom and Peters and Ms. Funk. (3) Non-CEO NEOs in 2021 were Messrs. Gulling, Olafson, Winterbottom and Peters. (4) Non-CEO NEOs in 2020 were Messrs. Gulling, Olafson and Winterbottom. | | |
| Peer Group Issuers, Footnote [Text Block] | The peer group consists of the S&P U.S. BMI Banks - Midwest Region Index. | | |
| PEO Total Compensation Amount | $ 1,262,648 | $ 1,024,797 | $ 967,524 |
| PEO Actually Paid Compensation Amount | 1,060,570 | 1,623,245 | 725,866 |
| Non-PEO NEO Average Total Compensation Amount | 905,793 | 765,149 | 731,858 |
| Non-PEO NEO Average Compensation Actually Paid Amount | 764,704 | 1,269,978 | 490,200 |
| Total Shareholder Return Amount | 112.28 | 131.21 | 78.82 |
| Peer Group Total Shareholder Return Amount | 98.03 | 113.59 | 85.98 |
| Net Income (Loss) | $ 46,399 | $ 49,607 | $ 32,712 |
| Company Selected Measure Amount | 0.2071 | 0.2033 | 0.1449 |
| PEO Name | Mr. Nelson. | Mr. Nelson. | Mr. Nelson. |
| Adjustment to PEO and Non-PEO NEO Compensation, Footnote | See the table immediately following these footnotes for a reconciliation of the Summary Compensation Table compensation and the Compensation Actually Paid to the CEO and Non-CEO NEOs. | | |
| PEO [Member] | Grant date fair value of stock awards granted in fiscal year [Member] | | | |
| Pay vs Performance Disclosure [Table] | | | |
| Adjustment to Compensation Amount | $ (395,589) | $ (257,325) | $ (188,198) |
| PEO [Member] | Fair value at fiscal year-end of outstanding and unvested stock awards granted in prior fiscal years [Member] | | | |
| Pay vs Performance Disclosure [Table] | | | |
| Adjustment to Compensation Amount | 319,725 | 407,213 | 261,150 |
| PEO [Member] | Change in fair value of outstanding and unvested stock awards granted in fiscal year [Member] | | | |
| Pay vs Performance Disclosure [Table] | | | |
| Adjustment to Compensation Amount | (151,725) | 361,410 | (178,260) |
| PEO [Member] | Change in fair value as of vesting date of stock awards granted in prior fiscal years for which applicable vesting conditions were satisfied during fiscal year [Member] | | | |
| Pay vs Performance Disclosure [Table] | | | |
| Adjustment to Compensation Amount | 25,511 | 87,150 | (136,350) |
| Non-PEO NEO [Member] | Grant date fair value of stock awards granted in fiscal year [Member] | | | |
| Pay vs Performance Disclosure [Table] | | | |
| Adjustment to Compensation Amount | (348,532) | (235,881) | (188,198) |
| Non-PEO NEO [Member] | Fair value at fiscal year-end of outstanding and unvested stock awards granted in prior fiscal years [Member] | | | |
| Pay vs Performance Disclosure [Table] | | | |
| Adjustment to Compensation Amount | 287,753 | 377,761 | 261,150 |
| Non-PEO NEO [Member] | Change in fair value of outstanding and unvested stock awards granted in fiscal year [Member] | | | |
| Pay vs Performance Disclosure [Table] | | | |
| Adjustment to Compensation Amount | (104,453) | 294,169 | (178,260) |
| Non-PEO NEO [Member] | Change in fair value as of vesting date of stock awards granted in prior fiscal years for which applicable vesting conditions were satisfied during fiscal year [Member] | | | |
| Pay vs Performance Disclosure [Table] | | | |
| Adjustment to Compensation Amount | $ 24,143 | $ 68,780 | $ (136,350) |
