Stock-Based Compensation - Narrative (Details) - USD ($)	9 Months Ended
Sep. 30, 2020	Sep. 30, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Plan vesting period	4 years
Plan exercisable period	10 years
Additional awards remaining available under plan (in shares)	921,000
Stock options granted (in shares)	0	0
Income tax benefit related to expense for share-based awards	$ 1,520,000	$ 1,960,000
Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense	0	0
Unrecognized compensation expense	0
Restricted Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation expense	$ 16,100,000
Recognition period for unrecognized compensation expense	2 years 8 months 12 days
Employees \| Restricted Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense	$ 5,570,000	7,400,000
Employees \| Restricted Stock Units \| Merger-related and other charges
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense related to acceleration of vesting		1,380,000
Compensation expense related to acquisition		740,000
Employees \| Restricted Stock Units \| Salaries and employee benefits expense
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense		5,280,000
Directors \| Restricted Stock Units \| Other operating expense
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense	$ 367,000	$ 283,000

Share-Based Compensation Arrangement By Share-Based Payment Award, Cost Related To Acquisition

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Amount of expense for award under share-based payment arrangement. Excludes amount capitalized.

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

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Weighted-average period over which cost not yet recognized is expected to be recognized for award under share-based payment arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

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Amount of tax benefit for recognition of expense of award under share-based payment arrangement.

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Amount of additional cost recognized for award under share-based payment arrangement from occurrence of event accelerating recognition of cost.

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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 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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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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