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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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Feb. 28, 2021 | Oct. 30, 2014 | Sep. 30, 2021 | Jun. 30, 2021 | Mar. 31, 2021 | Dec. 31, 2020 | Sep. 30, 2020 | Jun. 30, 2020 | Mar. 31, 2020 | Sep. 30, 2021 | Dec. 31, 2020 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015 | Jun. 30, 2017 | Oct. 31, 2014
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Common stock, voting par value |  |  | $ 0.01 |  |  | $ 0.01 |  |  |  | $ 0.01 | $ 0.01 |  |  |  |  |
Unrecognized stock option compensation expenses |  |  | $ 0 |  |  |  | $ 0 |  |  | $ 0 |  |  |  |  |  |
Common Stock |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stock Issued During Period Shares Stock Options Exercised |  |  | 25,866 | 11,031 | 55,908 | 49,138 | 165,375 | 5,196 | 55,402 |  |  |  |  |  |  |
Restricted Shares |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Unrecognized stock option compensation expenses |  |  | $ 7,840,000 |  |  |  | $ 5,566,000 |  |  | $ 7,840,000 |  |  |  |  |  |
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 |  |  | 698,331 |  |  |  |  |  |  | 698,331 |  |  |  |  |  |
Omnibus Plan | Restricted Shares |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Shares of restricted voting common stock granted |  |  |  |  |  |  |  |  |  | 308,452 |  |  |  |  |  |
Restricted shares vested |  |  |  |  |  |  |  |  |  | 103,187 | 113,264 |  |  |  |  |
Fair value of restricted shares, vested |  |  |  |  |  |  |  |  |  | $ 2,200,000 | $ 1,400,000 |  |  |  |  |
Fair value of unvested restricted stock awards |  |  | $ 14,200 |  |  |  |  |  |  | $ 14,200 |  |  |  |  |  |
Omnibus Plan | Restricted Shares | Common Stock |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Shares of restricted voting common stock granted |  |  |  |  |  |  |  |  |  | 308,452 |  |  |  |  |  |
Common stock, voting par value |  |  | $ 0.01 |  |  |  |  |  |  | $ 0.01 |  |  |  |  |  |
Restricted shares vested |  |  |  |  |  |  |  |  |  | 130 |  |  |  |  |  |
Omnibus Plan | Restricted Shares | Common Stock | Each Anniversary of Grant Date Over Four Years |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Shares vest on grant date |  |  |  |  |  |  |  |  |  | 119,344 |  |  |  |  |  |
Award vesting period |  |  |  |  |  |  |  |  |  | 4 years |  |  |  |  |  |
Omnibus Plan | Restricted Shares | Common Stock | Last Business Day of Grant Vest Over Three Years |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Shares vest on grant date |  |  |  |  |  |  |  |  |  | 109,475 |  |  |  |  |  |
Omnibus Plan | Restricted Shares | Common Stock | Each Anniversary of Grant Date Vest Over Three Years |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Shares vest on grant date |  |  |  |  |  |  |  |  |  | 38,279 |  |  |  |  |  |
Award vesting period |  |  |  |  |  |  |  |  |  | 3 years |  |  |  |  |  |
Omnibus Plan | Restricted Shares | Common Stock | Third Anniversary of Grant Date |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Shares vest on grant date |  |  |  |  |  |  |  |  |  | 9,141 |  |  |  |  |  |
Omnibus Plan | Performance-based Restricted Shares |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Shares of restricted voting common stock granted | 32,083 |  |  |  |  |  |  |  |  | 103,187 |  |  |  |  |  |
Period for number of shares earned under return on average assets | 3 years |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
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 |  |  | 1,344,548 |  |  | 1,390,579 |  |  |  | 1,344,548 | 1,390,579 |  |  |  |  |
Stock Issued During Period Shares Stock Options Exercised |  |  |  |  |  |  |  |  |  | 46,031 | 19,496 |  |  |  |  |
Proceeds from the exercise of stock options |  |  |  |  |  |  |  |  |  | $ 629,000,000 | $ 253,000,000 |  |  |  |  |
Tax benefit from exercise of stock options |  |  |  |  |  |  |  |  |  | $ 100,000,000 | $ 39,000,000 |  |  |  |  |
Options vested |  |  |  |  |  |  |  |  |  | 0 |  |  |  |  |  |
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 |  |  | 186,856 |  |  | 233,630 |  |  |  | 186,856 | 233,630 |  |  |  |  |
Stock Issued During Period Shares Stock Options Exercised |  |  |  |  |  |  |  |  |  | 46,774 | 255,615 |  |  |  |  |
Proceeds from the exercise of stock options |  |  |  |  |  |  |  |  |  | $ 540,000,000 | $ 2,800,000 |  |  |  |  |
Tax benefit from exercise of stock options |  |  |  |  |  |  |  |  |  | $ 87,000,000 | $ 219,000,000 |  |  |  |  |
Options vested |  |  | 0 |  |  |  |  |  |  | 0 |  |  |  |  |  |
Conversion calculation percentage |  |  | 4.725% |  |  |  |  |  |  | 4.725% |  |  |  |  |  |

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