Stockholders' Equity - Required Capital Amounts and Company's Respective Ratios (Detail) $ in Thousands	Sep. 30, 2021 USD ($)
Parent Company [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total Capital Amount	$ 1,140,098
Total Capital To Risk Weighted Assets Actual Ratio	17.57
Capital Required For Capital Adequacy Amount	$ 519,044
Capital Required For Capital Adequacy to Risk Weighted Assets Ratio	8.00
Capital With Capital Conservation Buffer Amount	$ 681,245
Capital With Capital Conservation To Risk Based Weighted Assets Buffer Ratio	10.50
Common Equity Tier 1 Risk Based Capital Amount	$ 973,762
Common Equity Tier 1 Risk Based Capital To Risk Weighted Assets Ratio	15.01
Common Equity Tier 1 Risk Based Capital Required For Capital Adequacy Amount	$ 291,962
Common Equity Tier 1 Risk Based Capital Required For Capital Adequacy To Risk Weighted Assets Ratio	4.50
Common Equity Tier 1 Risk Based Capital With Capital Conservation Buffer Amount	$ 454,163
Common Equity Tier 1 Risk Based Capital With Capital Conservation Buffer To Risk Weighted Assets Ratio	7.00
Tier 1 Risk Based Capital Amount	$ 999,762
Tier 1 Risk Based Capital To Risk Weighted Assets Ratio	15.41
Tier 1 Risk Based Capital Required For Capital Adequacy Amount	$ 389,283
Tier 1 Risk Based Capital Required For Capital Adequacy To Risk Weighted Assets Ratio	6.00
Tier 1 Risk Based Capital With Capital Conservation Buffer Amount	$ 551,484
Tier 1 Risk Based Capital With Capital Conservation Buffer To Risk Weighted Assets Ratio	8.50
Tier 1 Capital Amount	$ 999,762
Tier 1 Capital To Average Assets Ratio	9.06
Tier 1 Capital Required For Capital Adequacy Amount	$ 441,402
Tier 1 Capital Required For Capital Adequacy To Average Assets Ratio	4.00
BancFirst [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total Capital Amount	$ 979,699
Total Capital To Risk Weighted Assets Actual Ratio	16.77
Capital Required For Capital Adequacy Amount	$ 467,478
Capital Required For Capital Adequacy to Risk Weighted Assets Ratio	8.00
Capital With Capital Conservation Buffer Amount	$ 613,565
Capital With Capital Conservation To Risk Based Weighted Assets Buffer Ratio	10.50
Capital Required To Be Well Capitalized Amount	$ 584,348
Capital Required To Be Well Capitalized To Risk Weighted Assets Ratio	10.00
Common Equity Tier 1 Risk Based Capital Amount	$ 886,552
Common Equity Tier 1 Risk Based Capital To Risk Weighted Assets Ratio	15.17
Common Equity Tier 1 Risk Based Capital Required For Capital Adequacy Amount	$ 262,957
Common Equity Tier 1 Risk Based Capital Required For Capital Adequacy To Risk Weighted Assets Ratio	4.50
Common Equity Tier 1 Risk Based Capital With Capital Conservation Buffer Amount	$ 409,043
Common Equity Tier 1 Risk Based Capital With Capital Conservation Buffer To Risk Weighted Assets Ratio	7.00
Common Equity Tier 1 Risk Based Capital Required To Be Well Capitalized Amount	$ 379,826
Common Equity Tier 1 Risk Based Capital Required To Be Well Capitalized To Risk Weighted Assets Ratio	6.50
Tier 1 Risk Based Capital Amount	$ 906,552
Tier 1 Risk Based Capital To Risk Weighted Assets Ratio	15.51
Tier 1 Risk Based Capital Required For Capital Adequacy Amount	$ 350,609
Tier 1 Risk Based Capital Required For Capital Adequacy To Risk Weighted Assets Ratio	6.00
Tier 1 Risk Based Capital With Capital Conservation Buffer Amount	$ 496,696
Tier 1 Risk Based Capital With Capital Conservation Buffer To Risk Weighted Assets Ratio	8.50
Tier 1 Risk Based Capital Required To Be Well Capitalized Amount	$ 467,478
Tier 1 Risk Based Capital Required To Be Well Capitalized To Risk Weighted Assets Ratio	8.00
Tier 1 Capital Amount	$ 906,552
Tier 1 Capital To Average Assets Ratio	9.06
Tier 1 Capital Required For Capital Adequacy Amount	$ 400,265
Tier 1 Capital Required For Capital Adequacy To Average Assets Ratio	4.00
Tier 1 Capital Required To Be Well Capitalized Amount	$ 500,331
Tier 1 Capital Required To Be Well Capitalized To Average Assets Ratio	5.00
Pegasus Bank [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total Capital Amount	$ 80,244
Total Capital To Risk Weighted Assets Actual Ratio	12.53
Capital Required For Capital Adequacy Amount	$ 51,251
Capital Required For Capital Adequacy to Risk Weighted Assets Ratio	8.00
Capital With Capital Conservation Buffer Amount	$ 67,267
Capital With Capital Conservation To Risk Based Weighted Assets Buffer Ratio	10.50
Capital Required To Be Well Capitalized Amount	$ 64,064
Capital Required To Be Well Capitalized To Risk Weighted Assets Ratio	10.00
Common Equity Tier 1 Risk Based Capital Amount	$ 74,911
Common Equity Tier 1 Risk Based Capital To Risk Weighted Assets Ratio	11.69
Common Equity Tier 1 Risk Based Capital Required For Capital Adequacy Amount	$ 28,829
Common Equity Tier 1 Risk Based Capital Required For Capital Adequacy To Risk Weighted Assets Ratio	4.50
Common Equity Tier 1 Risk Based Capital With Capital Conservation Buffer Amount	$ 44,845
Common Equity Tier 1 Risk Based Capital With Capital Conservation Buffer To Risk Weighted Assets Ratio	7.00
Common Equity Tier 1 Risk Based Capital Required To Be Well Capitalized Amount	$ 41,642
Common Equity Tier 1 Risk Based Capital Required To Be Well Capitalized To Risk Weighted Assets Ratio	6.50
Tier 1 Risk Based Capital Amount	$ 74,911
Tier 1 Risk Based Capital To Risk Weighted Assets Ratio	11.69
Tier 1 Risk Based Capital Required For Capital Adequacy Amount	$ 38,438
Tier 1 Risk Based Capital Required For Capital Adequacy To Risk Weighted Assets Ratio	6.00
Tier 1 Risk Based Capital With Capital Conservation Buffer Amount	$ 54,454
Tier 1 Risk Based Capital With Capital Conservation Buffer To Risk Weighted Assets Ratio	8.50
Tier 1 Risk Based Capital Required To Be Well Capitalized Amount	$ 51,251
Tier 1 Risk Based Capital Required To Be Well Capitalized To Risk Weighted Assets Ratio	8.00
Tier 1 Capital Amount	$ 74,911
Tier 1 Capital To Average Assets Ratio	7.28
Tier 1 Capital Required For Capital Adequacy Amount	$ 41,173
Tier 1 Capital Required For Capital Adequacy To Average Assets Ratio	4.00
Tier 1 Capital Required To Be Well Capitalized Amount	$ 51,466
Tier 1 Capital Required To Be Well Capitalized To Average Assets Ratio	5.00