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Fair value of unvested restricted stock awards.

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Share based compensation arrangement by share based payment award earned under return on average assets in period.

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Share based compensation arrangement by share based payment award equity instruments other than options expected to vest grants in period.

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Aggregate number of common shares reserved for future issuance.

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Face amount or stated value per share of common stock.

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Amount of cost not yet recognized for nonvested award under share-based payment arrangement.

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Amount of tax benefit from exercise of option under share-based payment arrangement.

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Amount of cash inflow from exercise of option under share-based payment arrangement.

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Period over which grantee's right to exercise award under share-based payment arrangement is no longer contingent on satisfaction of service or performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. Includes, but is not limited to, combination of market, performance or service condition.

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The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The number of equity-based payment instruments, excluding stock (or unit) options, that vested during the reporting period.

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Fair value of share-based awards for which the grantee gained the right by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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The difference between the maximum number of shares (or other type of equity) authorized for issuance under the plan (including the effects of amendments and adjustments), and the sum of: 1) the number of shares (or other type of equity) already issued upon exercise of options or other equity-based awards under the plan; and 2) shares (or other type of equity) reserved for issuance on granting of outstanding awards, net of cancellations and forfeitures, if applicable.

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Gross number of share options (or share units) granted during the period.

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Number of options outstanding, including both vested and non-vested options.

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Period from grant date that an equity-based award expires, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Number of options vested.

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Number of share options (or share units) exercised during the current period.

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Share-Based Compensation - Additional Information (Details) - USD ($)	1 Months Ended		3 Months Ended							9 Months Ended	12 Months Ended
	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%