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Share-Based Compensation - Additional Information (Details) - USD ($) |  |  |  |  | 1 Months Ended | 3 Months Ended | 9 Months Ended | 12 Months Ended |
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May 15, 2019 | Apr. 30, 2019 | Oct. 03, 2017 | Jul. 06, 2017 | Feb. 29, 2020 | Oct. 31, 2014 | Sep. 30, 2020 | Jun. 30, 2020 | Mar. 31, 2020 | Dec. 31, 2019 | Sep. 30, 2019 | Jun. 30, 2019 | Mar. 31, 2019 | Sep. 30, 2020 | Sep. 30, 2019 | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015 | Jun. 30, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Common stock, voting par value |  |  |  |  |  |  | $ 0.01 |  |  | $ 0.01 |  |  |  | $ 0.01 |  | $ 0.01 |  |  |  |  |
Proceeds from the exercise of stock options |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 2,548,000 | $ 2,359,000 |  |  |  |  |  |
Oak Park River Forest Bankshares, Inc. |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Consideration paid in cash | $ 4,200,000 | $ 6,163,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Business combination conversion of outstanding options | 35,870 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Performance Options Grants |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Options vested |  |  | 414,894 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Common Stock |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Issuance of common stock in connection with restricted stock awards, shares |  |  |  |  |  |  | (5,886) |  | 174,036 | 16,345 | 36,350 | 136,469 | (8,500) |  |  |  |  |  |  |  |
Number of options exercised |  |  |  |  |  |  | 165,375 | 5,196 | 55,402 | 71,029 | 3,472 | 116,048 | 50,662 |  |  |  |  |  |  |  |
Common Stock | Restricted Shares |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Shares of restricted voting common stock granted |  |  |  |  | 175,493 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Common stock, voting par value |  |  |  |  | $ 0.01 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Shares vested |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 5,254 |  |  |  |  |
Common Stock | Restricted Shares | Each Anniversary of Grant Date Over Four Years |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Award vesting period |  |  |  |  | 4 years |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Shares vest on grant date |  |  |  |  | 104,779 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Common Stock | Restricted Shares | Each Anniversary of Grant Date Vest Over Three Years |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Award vesting period |  |  |  |  | 3 years |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Shares vest on grant date |  |  |  |  | 38,786 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Omnibus Plan |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of common stock reserved for issuance |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 1,550,000
Number of common shares available for future grants |  |  |  |  |  |  | 1,001,731 |  |  |  |  |  |  | 1,001,731 |  |  |  |  |  |  |
Omnibus Plan | Restricted Shares |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Award vesting period |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 4 years |  |  |
Shares of restricted voting common stock granted |  |  |  |  |  |  |  |  |  |  |  |  |  | 175,493 |  |  |  |  |  |  |
Restricted shares vested |  |  |  |  |  |  |  |  |  |  |  |  |  | 104,364 |  | 48,491 |  |  |  |  |
Fair value of restricted shares, vested |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 1,200,000 |  | $ 900,000 |  |  |  |  |
Fair value of unvested restricted stock awards |  |  |  |  |  |  | $ 4,400,000 |  |  |  |  |  |  | $ 4,400,000 |  |  |  |  |  |  |
Omnibus Plan | Performance-based Restricted Shares |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Shares of restricted voting common stock granted |  |  |  |  | 31,928 |  |  |  |  |  |  |  |  |  |  | 20,975 | 11,165 |  |  |  |
Period for number of shares earned under return on average assets |  |  |  |  | 3 years |  |  |  |  |  |  |  |  |  |  | 3 years | 3 years |  |  |  |
Minimum target percentage in assets of peer group consisting publicly-traded bank holding companies |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 50.00% |  |  |  |
Maximum target percentage in assets of peer group consisting publicly-traded bank holding companies |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 200.00% |  |  |  |
Omnibus Plan | Performance-based Restricted Shares | Threshold performance |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of target number of shares to be earned |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 25.00% |  |  |  |
Omnibus Plan | Performance-based Restricted Shares | 50th Percentile Performance |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of target number of shares to be earned |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 100.00% |  |  |  |
Omnibus Plan | Performance-based Restricted Shares | 75th Percentile Performance |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of target number of shares to be earned |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 125.00% |  |  |  |
Omnibus Plan | Common Stock |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Issuance of common stock in connection with restricted stock awards, shares |  |  |  | 58,900 |  |  |  |  |  |  |  |  |  |  |  |  |  | 11,898 |  |  |
Omnibus Plan | Common Stock | Restricted Shares |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Shares of restricted voting common stock granted |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 210,622 | 131,157 |  |  |  |
Common stock, voting par value |  |  |  |  |  |  |  |  |  | $ 0.01 |  |  |  |  |  | $ 0.01 | $ 0.01 |  |  |  |
Shares vested |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 4,053 |  |  |  |
Omnibus Plan | Common Stock | Restricted Shares | Each Anniversary of Grant Date Over Four Years |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Award vesting period |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 4 years | 4 years |  |  |  |
Shares vest on grant date |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 111,823 | 101,559 |  |  |  |
Omnibus Plan | Common Stock | Restricted Shares | Each Anniversary of Grant Date Vest Over Three Years |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Award vesting period |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 3 years | 3 years |  |  |  |
Shares vest on grant date |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 72,570 | 14,380 |  |  |  |
BYB Plan |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of common shares available for future grants |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 0
Number of options granted |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 0 | 212,400 | 1,634,568 |
Number of shares outstanding |  |  |  |  |  |  | 1,390,579 |  |  | 1,410,075 |  |  |  | 1,390,579 |  | 1,410,075 |  |  |  |  |
Options vested |  |  |  |  |  |  |  |  |  |  |  |  |  | 20,000 |  |  |  |  |  |  |
Number of options exercised |  |  |  |  |  |  |  |  |  |  |  |  |  | 19,496 |  | 127,997 |  |  |  |  |
Proceeds from the exercise of stock options |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 253,000 |  | $ 1,900,000 |  |  |  |  |
Tax benefit from exercise of stock options |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 39,000 |  | $ 145,000 |  |  |  |  |
BYB Plan | Maximum |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of common shares available for future grants |  |  |  |  |  | 2,476,122 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
BYB Plan | Time Options Grants |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Award contractual term |  |  |  |  |  | 10 years |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
BYB Plan | Time Options Grants | Maximum |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Award vesting period |  |  |  |  |  | 5 years |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
BYB Plan | Time Options Grants | Minimum |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Award vesting period |  |  |  |  |  | 1 year |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
BYB Plan | Performance Options Grants |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Award contractual term |  |  |  |  |  | 10 years |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
BYB Plan | Performance Options Grants | Maximum |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Award vesting period |  |  |  |  |  | 5 years |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
BYB Plan | Performance Options Grants | Minimum |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Award vesting period |  |  |  |  |  | 1 year |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
FEB Plan |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of shares outstanding |  |  |  |  |  |  | 285,792 |  |  | 511,169 |  |  |  | 285,792 |  | 511,169 |  |  |  |  |
Number of options exercised |  |  |  |  |  |  |  |  |  |  |  |  |  | 206,477 |  | 113,214 |  |  |  |  |
Proceeds from the exercise of stock options |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 2,300,000 |  | $ 1,300,000 |  |  |  |  |
Tax benefit from exercise of stock options |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 165,000 |  | $ 253,000 |  |  |  |  |
Conversion calculation percentage |  |  |  |  |  |  | 472.50% |  |  |  |  |  |  | 472.50% |  |  |  |  |  |  |

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