| Pay vs Performance Disclosure - USD ($) | 12 Months Ended |
| Dec. 31, 2022 | Dec. 31, 2021 | Dec. 31, 2020 |
| Pay vs Performance Disclosure [Table] | | | |
| Pay vs Performance [Table Text Block] | Year Summary Compensation Table Total for Bruce K. Lee (1) ($) Compensation Actually Paid to Bruce K. Lee 1, 2, 3 ($) Average Summary Compensation Table Total for Non-PEO NEOs (1) ($) Average Compensation Actually Paid to Non-PEO NEOs 1, 2, 3 ($) Value of Initial Fixed $100 Investment based on: (4) Net Income PPNR (5) ($ Millions) TSR ($) Peer Group TSR (a) (b) (c) (d) (e) (f) (g) (h) (i) 2022 $ 3,055,878 $ 2,675,277 $ 1,042,299 $ 1,134,119 $ 100.07 $ 97.52 $ 212.2 $ 295.7 2021 $ 2,852,290 $ 3,100,827 $ 1,041,819 $ 1,112,786 $ 106.17 $ 124.06 $ 219.9 $ 265.7 2020 $ 2,245,882 $ 2,372,655 $ 854,562 $ 866,117 $ 83.03 $ 89.68 $ 137.9 $ 241.1 (1) Bruce K. Lee was our PEO for each year presented. The individuals comprising the Non-PEO NEOs for each year are as follows: 2022-Bryan R. McKeag, Jay L. Kim, David A. Prince, and Kevin G. Quinn; 2021-Lynn B. Fuller, Bryan R. McKeag, David A. Prince, and Daniel C. Stevens; 2020-Lynn B. Fuller, Bryan R. McKeag, David A. Prince, and Daniel C. Stevens. (2) The amounts shown for Compensation Actually Paid have been recalculated in accordance with Item 402(v) of Regulation S-K and do not reflect compensation actually earned, realized or received by the Company’s NEOs. These amounts reflect the Summary Compensation Table Total with certain adjustments as described in footnote 3 below. (3) Compensation Actually Paid reflects the exclusions and inclusions of certain amounts for the PEO and the Non-PEO NEOs as set forth below. Amounts shown in the Exclusion of Stock Awards and Option Awards columns below are the totals from the Stock Awards and Option Awards columns set forth in the Summary Compensation Table contained in our Proxy Statement for the respective reporting year. Amounts in the Inclusion of Equity Values below are not reported elsewhere and have been calculated in accordance with FASB ASC Topic 718. | | |
| Named Executive Officers, Footnote [Text Block] | (1) Bruce K. Lee was our PEO for each year presented. The individuals comprising the Non-PEO NEOs for each year are as follows: 2022-Bryan R. McKeag, Jay L. Kim, David A. Prince, and Kevin G. Quinn; 2021-Lynn B. Fuller, Bryan R. McKeag, David A. Prince, and Daniel C. Stevens; 2020-Lynn B. Fuller, Bryan R. McKeag, David A. Prince, and Daniel C. Stevens. | | |
| Peer Group Issuers, Footnote [Text Block] | (4) The Peer Group TSR set forth in this table utilizes the KBW Bank Index, which we also utilize in the stock performance graph required by Item 201(e) of Regulation S-K included in our Annual Report on Form 10-K for the year ended December 31, 2022. The comparison assumes $100 was invested for the period starting December 31, 2019, through the end of the listed year in the Company and in the KBW Nasdaq Bank Index, respectively. Historical stock performance in not necessarily indicative of future stock performance. | | |
| PEO Total Compensation Amount | $ 3,055,878 | $ 2,852,290 | $ 2,245,882 |
| PEO Actually Paid Compensation Amount | $ 2,675,277 | 3,100,827 | 2,372,655 |
| Adjustment To PEO Compensation, Footnote [Text Block] | (3) Compensation Actually Paid reflects the exclusions and inclusions of certain amounts for the PEO and the Non-PEO NEOs as set forth below. Amounts shown in the Exclusion of Stock Awards and Option Awards columns below are the totals from the Stock Awards and Option Awards columns set forth in the Summary Compensation Table contained in our Proxy Statement for the respective reporting year. Amounts in the Inclusion of Equity Values below are not reported elsewhere and have been calculated in accordance with FASB ASC Topic 718. Year Summary Compensation Table Total for Exclusion of Stock Awards Inclusion of Equity Values for Bruce K. Lee Compensation Actually Paid to Bruce K. Lee 2022 $ 3,055,878 $ (1,286,285) $ 905,684 $ 2,675,277 2021 $ 2,852,290 $ (1,224,086) $ 1,472,623 $ 3,100,827 2020 $ 2,245,882 $ (823,760) $ 950,533 $ 2,372,655 The amounts in the Inclusion of Equity Values in the tables above are derived from the amounts set forth in the following tables: Year Year-End Fair Value of Equity Change in Fair Value from Last Change in Fair Value from Last Total-Inclusion of Equity Values 2022 $ 1,165,725 $ (260,018) $ (23) $ 905,684 2021 $ 1,184,798 $ 187,350 $ 100,475 $ 1,472,623 2020 $ 990,077 $ 460 $ (40,004) $ 950,533 | | |
| Non-PEO NEO Average Total Compensation Amount | $ 1,042,299 | 1,041,819 | 854,562 |
| Non-PEO NEO Average Compensation Actually Paid Amount | $ 1,134,119 | 1,112,786 | 866,117 |
| Adjustment to Non-PEO NEO Compensation Footnote [Text Block] | (3) Compensation Actually Paid reflects the exclusions and inclusions of certain amounts for the PEO and the Non-PEO NEOs as set forth below. Amounts shown in the Exclusion of Stock Awards and Option Awards columns below are the totals from the Stock Awards and Option Awards columns set forth in the Summary Compensation Table contained in our Proxy Statement for the respective reporting year. Amounts in the Inclusion of Equity Values below are not reported elsewhere and have been calculated in accordance with FASB ASC Topic 718. Year Average Summary Compensation Table Average Exclusion of Stock Awards and Option Awards for Non-PEO NEOs Average Inclusion of Equity Values for Non-PEO NEOs Average Compensation Actually Paid to Non-PEO NEOs 2022 $ 1,042,299 $ (363,444) $ 455,264 $ 1,134,119 2021 $ 1,041,819 $ (341,049) $ 412,016 $ 1,112,786 2020 $ 854,562 $ (187,636) $ 199,191 $ 866,117 Year Average Year-End Fair Value of Average Change in Fair Value Average Change in Fair Value Total-Average Inclusion 2022 $ 500,718 $ (46,425) $ 971 $ 455,264 2021 $ 330,103 $ 44,002 $ 37,911 $ 412,016 2020 $ 225,519 $ (5,286) $ (21,042) $ 199,191 | | |
| Compensation Actually Paid vs. Total Shareholder Return [Text Block] | The following chart sets forth the relationship between Compensation Actually Paid to our PEO, the average of Compensation Actually Paid to our non-PEO NEOs, and the Company’s cumulative TSR over the three most recently completed fiscal years. | | |
| Compensation Actually Paid vs. Net Income [Text Block] | The following chart sets forth the relationship between Compensation Actually Paid to our PEO, the average of Compensation Actually Paid to our Non-PEO NEOs, and our net income during the three most recently completed fiscal years. | | |
| Compensation Actually Paid vs. Company Selected Measure [Text Block] | The following chart sets forth the relationship between Compensation Actually Paid to our PEO, the average of the Compensation Actually Paid to our Non-PEO NEOs, and our PPNR during the three most recently completed fiscal years. | | |
| Total Shareholder Return Vs Peer Group [Text Block] | The following chart compares our cumulative TSR over the three most recently completed fiscal years to that of the KBW Nasdaq Bank Index over the same period. | | |
| Tabular List [Table Text Block] | * EPS Growth * Loan Growth * Noninterest Income * PPNR * Return on Average Assets * Total Stockholder Return | | |
| Total Shareholder Return Amount | $ 100.07 | 106.17 | 83.03 |
| Peer Group Total Shareholder Return Amount | 97.52 | 124.06 | 89.68 |
| Net Income (Loss) | $ 212,200,000 | $ 219,900,000 | $ 137,900,000 |
| Company Selected Measure Amount | 295,700,000 | 265,700,000 | 241,100,000 |
| PEO Name | Bruce K. Lee | | |
| Additional 402(v) Disclosure [Text Block] | (2) The amounts shown for Compensation Actually Paid have been recalculated in accordance with Item 402(v) of Regulation S-K and do not reflect compensation actually earned, realized or received by the Company’s NEOs. These amounts reflect the Summary Compensation Table Total with certain adjustments as described in footnote 3 below.In calculating the Inclusion of Equity Values in the preceding tables, we value performance awards with a market condition using a Monte Carlo simulation model; this methodology differs from the manner in which we calculated the value on the date of grant in the relevant Grants of Plan-Based Awards tables and Summary Compensation Tables. | | |
| Measure [Axis]: 1 | | | |
| Pay vs Performance Disclosure [Table] | | | |
| Measure Name | EPS Growth | | |
| Measure [Axis]: 2 | | | |
| Pay vs Performance Disclosure [Table] | | | |
| Measure Name | Loan Growth | | |
| Measure [Axis]: 3 | | | |
| Pay vs Performance Disclosure [Table] | | | |
| Measure Name | Noninterest Income | | |
| Measure [Axis]: 4 | | | |
| Pay vs Performance Disclosure [Table] | | | |
| Measure Name | PPNR | | |
| Non-GAAP Measure Description [Text Block] | (5) We determined adjusted core pre-provision pre-tax net revenue (“PPNR”) to be the most important financial performance measure used to link Company performance to Compensation Actually Paid to our PEO and Non-PEO NEOs in 2022. See the reconciliation of our comparable GAAP financial measures to PPNR in the table below. The table below shows how PPNR was calculated, dollars in thousands: Reconciliation of Non-GAAP Measure-Adjusted Core Pre-Provision Pre-Tax Net Revenue For the Year Ended December 31, 2022 2021 2020 Net income (GAAP) $ 212,280 $ 219,923 $ 137,938 Provision (benefit) for credit losses 15,370 (17,575) 67,066 Income taxes 55,573 55,335 36,053 Pre-provision pre-tax revenue 283,223 257,683 241,057 Partnership investment in tax credit projects 5,040 6,303 — Acquisition costs 7,586 — — IT project write-offs — 1,743 — Adjusted core pre-provision pre-tax net revenue (non-GAAP) $ 295,849 $ 265,729 $ 241,057 | | |
| Measure [Axis]: 5 | | | |
| Pay vs Performance Disclosure [Table] | | | |
| Measure Name | Return on Average Assets | | |
| Measure [Axis]: 6 | | | |
| Pay vs Performance Disclosure [Table] | | | |
| Measure Name | Total Stockholder Return | | |
| PEO [Member] | Stock and Option Awards Adjustments [Member] | | | |
| Pay vs Performance Disclosure [Table] | | | |
| Adjustment to Compensation Amount | $ (1,286,285) | $ (1,224,086) | $ (823,760) |
| PEO [Member] | Equity Award Adjustments [Member] | | | |
| Pay vs Performance Disclosure [Table] | | | |
| Adjustment to Compensation Amount | 905,684 | 1,472,623 | 950,533 |
| PEO [Member] | Equity Awards Granted During the Year, Unvested [Member] | | | |
| Pay vs Performance Disclosure [Table] | | | |
| Adjustment to Compensation Amount, Equity Awards | 1,165,725 | 1,184,798 | 990,077 |
| PEO [Member] | Equity Awards Granted in Prior Years, Unvested [Member] | | | |
| Pay vs Performance Disclosure [Table] | | | |
| Adjustment to Compensation Amount, Equity Awards | (260,018) | 187,350 | 460 |
| PEO [Member] | Equity Awards Granted in Prior Years, Vested [Member] | | | |
| Pay vs Performance Disclosure [Table] | | | |
| Adjustment to Compensation Amount, Equity Awards | (23) | 100,475 | (40,004) |
| Non-PEO NEO [Member] | Stock and Option Awards Adjustments [Member] | | | |
| Pay vs Performance Disclosure [Table] | | | |
| Adjustment to Compensation Amount | (363,444) | (341,049) | (187,636) |
| Non-PEO NEO [Member] | Equity Award Adjustments [Member] | | | |
| Pay vs Performance Disclosure [Table] | | | |
| Adjustment to Compensation Amount | 455,264 | 412,016 | 199,191 |
| Non-PEO NEO [Member] | Equity Awards Granted During the Year, Unvested [Member] | | | |
| Pay vs Performance Disclosure [Table] | | | |
| Adjustment to Compensation Amount, Equity Awards | 500,718 | 330,103 | 225,519 |
| Non-PEO NEO [Member] | Equity Awards Granted in Prior Years, Unvested [Member] | | | |
| Pay vs Performance Disclosure [Table] | | | |
| Adjustment to Compensation Amount, Equity Awards | (46,425) | 44,002 | (5,286) |
| Non-PEO NEO [Member] | Equity Awards Granted in Prior Years, Vested [Member] | | | |
| Pay vs Performance Disclosure [Table] | | | |
| Adjustment to Compensation Amount, Equity Awards | $ 971 | $ 37,911 | $ (21,042) |
