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Loans Held for Investment and Allowance for Credit Losses on Loans - Additional Information (Detail) - USD ($) |  | 3 Months Ended | 6 Months Ended | 12 Months Ended |
--- | --- | --- | --- | --- | ---
Jan. 01, 2020 | Jun. 30, 2021 | Jun. 30, 2020 | Jun. 30, 2021 | Jun. 30, 2020 | Dec. 31, 2020 | May 20, 2021
Financing Receivable Recorded Investment [Line Items] |  |  |  |  |  |  |
Paycheck protection program amortized processing fees recognized as interest income |  | $ 21,700,000 | $ 3,700,000 | $ 11,900,000 | $ 3,700,000 |  |
Total loans |  | 6,191,230,000 |  | 6,191,230,000 |  | $ 6,394,506,000 |
Gains (losses) on sale of property held in other real estate owned |  |  |  | (105,000) | 2,300,000 |  |
Interest income that would have been recognized |  |  |  | 1,200 | 983,000,000 |  |
Decrease in allowance for credit losses upon adoption of ASC 326 | $ 3,200,000 |  |  |  | 3,195,000 |  |
Commercial Real Estate |  |  |  |  |  |  |
Financing Receivable Recorded Investment [Line Items] |  |  |  |  |  |  |
Loans modification |  | $ 61,800,000 |  | 61,800,000 |  | 81,700,000 |
U.S. Federal Agencies [Member] |  |  |  |  |  |  |
Financing Receivable Recorded Investment [Line Items] |  |  |  |  |  |  |
Nonaccrual loans guaranteed by government agencies |  |  |  | 3,500,000 |  | 7,800,000 |
Previous Corporate Headquarters [Member] | Other Real Estate Owned [Member] |  |  |  |  |  |  |
Financing Receivable Recorded Investment [Line Items] |  |  |  |  |  |  |
Transferred premises and equipment to other real estate |  |  |  | 2,400,000 |  |  |
First National Bank and Trust [Member] |  |  |  |  |  |  |
Financing Receivable Recorded Investment [Line Items] |  |  |  |  |  |  |
Total loans |  |  |  |  |  |  | $ 195,000,000
First National Bank and Trust [Member] | Oklahoma [Member] |  |  |  |  |  |  |
Financing Receivable Recorded Investment [Line Items] |  |  |  |  |  |  |
Other real estate owned purchased |  |  |  | $ 2,500,000 |  |  |
BancFirst’s [Member] |  |  |  |  |  |  |
Financing Receivable Recorded Investment [Line Items] |  |  |  |  |  |  |
Percentage of loans secured by real estate |  | 56.00% |  | 56.00% |  |  |
Pegasus Bank [Member] |  |  |  |  |  |  |
Financing Receivable Recorded Investment [Line Items] |  |  |  |  |  |  |
Total loans |  | $ 586,043,000 |  | $ 586,043,000 |  | 554,548,000 |
Decrease in allowance for credit losses upon adoption of ASC 326 |  |  |  |  | $ (2,488,000) |  |
Pegasus Bank [Member] | Real Estate [Member] |  |  |  |  |  |  |
Financing Receivable Recorded Investment [Line Items] |  |  |  |  |  |  |
Percentage of loans secured by real estate |  | 51.00% |  | 51.00% |  |  |
Total loans |  | $ 297,000,000 |  | $ 297,000,000 |  |  |
Pegasus Bank [Member] | Commercial and Agricultural Non-Real Estate [Member] |  |  |  |  |  |  |
Financing Receivable Recorded Investment [Line Items] |  |  |  |  |  |  |
Total loans |  | $ 268,000,000 |  | $ 268,000,000 |  | $ 262,000,000 |

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