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Loans Held for Investment and Allowance for Credit Losses on Loans - Additional Information (Detail) - USD ($) | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2021 | Mar. 31, 2020 | Dec. 31, 2020
Financing Receivable Recorded Investment [Line Items] |  |  |
Total loans | $ 6,358,390,000 |  | $ 6,394,506,000
Paycheck protection program unamortized processing fees recognized as interest income | 9,800,000 |  |
Gains (losses) on sale of property held in other real estate owned | (55,000) | $ 2,200,000 |
Interest income that would have been recognized | 521,000,000 | 332,000,000 |
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 | 88,000,000 |  | 81,700,000
U.S. Federal Agencies [Member] |  |  |
Financing Receivable Recorded Investment [Line Items] |  |  |
Nonaccrual loans guaranteed by government agencies | $ 6,600,000 |  | 7,800,000
BancFirst’s [Member] |  |  |
Financing Receivable Recorded Investment [Line Items] |  |  |
Percentage of loans secured by real estate | 52.00% |  |
BancFirst’s [Member] | Commercial and Agricultural Non-Real Estate [Member] | Ancillary Services [Member] |  |  |
Financing Receivable Recorded Investment [Line Items] |  |  |
Total loans | $ 67,000,000 |  | 59,000,000
BancFirst’s [Member] | Energy [Member] |  |  |
Financing Receivable Recorded Investment [Line Items] |  |  |
Total loans | 183,000,000 |  | 191,000,000
BancFirst’s [Member] | Energy [Member] | Midstream Energy Loans |  |  |
Financing Receivable Recorded Investment [Line Items] |  |  |
Total loans | 33,000,000 |  | 50,000,000
Pegasus Bank [Member] |  |  |
Financing Receivable Recorded Investment [Line Items] |  |  |
Total loans | 544,493,000 |  | 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] |  |  |
Total loans | $ 294,000,000 |  |
Percentage of loans secured by real estate | 54.00% |  |
Pegasus Bank [Member] | Commercial and Agricultural Non-Real Estate [Member] |  |  |
Financing Receivable Recorded Investment [Line Items] |  |  |
Total loans | $ 228,000,000 |  | 262,000,000
Pegasus Bank [Member] | Energy [Member] |  |  |
Financing Receivable Recorded Investment [Line Items] |  |  |
Total loans | 109,000,000 |  | 107,000,000
Pegasus Bank [Member] | Energy [Member] | Ancillary Services [Member] |  |  |
Financing Receivable Recorded Investment [Line Items] |  |  |
Total loans | 15,000,000 |  | 13,000,000
Pegasus Bank [Member] | Energy [Member] | Midstream Energy Loans |  |  |
Financing Receivable Recorded Investment [Line Items] |  |  |
Total loans | $ 11,000,000 |  | $ 11,000,000

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