Stockholders' Equity - Required Capital Amounts and Company's Respective Ratios (Detail) $ in Thousands	Sep. 30, 2020 USD ($)
Parent Company [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total Capital Amount	$ 971,248
Total Capital To Risk Weighted Assets Actual Ratio	0.1523
Capital Required For Capital Adequacy Amount	$ 510,168
Capital Required For Capital Adequacy to Risk Weighted Assets Ratio	0.0800
Capital With Capital Conservation Buffer Amount	$ 669,595
Capital With Capital Conservation To Risk Based Weighted Assets Buffer Ratio	0.1050
Common Equity Tier 1 Risk Based Capital Amount	$ 865,208
Common Equity Tier 1 Risk Based Capital To Risk Weighted Assets Ratio	0.1357
Common Equity Tier 1 Risk Based Capital Required For Capital Adequacy Amount	$ 286,969
Common Equity Tier 1 Risk Based Capital Required For Capital Adequacy To Risk Weighted Assets Ratio	0.0450
Common Equity Tier 1 Risk Based Capital With Capital Conservation Buffer Amount	$ 446,397
Common Equity Tier 1 Risk Based Capital With Capital Conservation Buffer To Risk Weighted Assets Ratio	0.0700
Tier 1 Risk Based Capital Amount	$ 891,208
Tier 1 Risk Based Capital To Risk Weighted Assets Ratio	0.1398
Tier 1 Risk Based Capital Required For Capital Adequacy Amount	$ 382,626
Tier 1 Risk Based Capital Required For Capital Adequacy To Risk Weighted Assets Ratio	0.0600
Tier 1 Risk Based Capital With Capital Conservation Buffer Amount	$ 542,053
Tier 1 Risk Based Capital With Capital Conservation Buffer To Risk Weighted Assets Ratio	0.0850
Tier 1 Capital Amount	$ 891,208
Tier 1 Capital To Average Assets Ratio	0.0943
Tier 1 Capital Required For Capital Adequacy Amount	$ 377,862
Tier 1 Capital Required For Capital Adequacy To Average Assets Ratio	0.0400
BancFirst [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total Capital Amount	$ 878,442
Total Capital To Risk Weighted Assets Actual Ratio	0.1510
Capital Required For Capital Adequacy Amount	$ 465,403
Capital Required For Capital Adequacy to Risk Weighted Assets Ratio	0.0800
Capital With Capital Conservation Buffer Amount	$ 610,841
Capital With Capital Conservation To Risk Based Weighted Assets Buffer Ratio	0.1050
Capital Required To Be Well Capitalized Amount	$ 581,754
Capital Required To Be Well Capitalized To Risk Weighted Assets Ratio	0.1000
Common Equity Tier 1 Risk Based Capital Amount	$ 785,357
Common Equity Tier 1 Risk Based Capital To Risk Weighted Assets Ratio	0.1350
Common Equity Tier 1 Risk Based Capital Required For Capital Adequacy Amount	$ 261,789
Common Equity Tier 1 Risk Based Capital Required For Capital Adequacy To Risk Weighted Assets Ratio	0.0450
Common Equity Tier 1 Risk Based Capital With Capital Conservation Buffer Amount	$ 407,227
Common Equity Tier 1 Risk Based Capital With Capital Conservation Buffer To Risk Weighted Assets Ratio	0.0700
Common Equity Tier 1 Risk Based Capital Required To Be Well Capitalized Amount	$ 378,140
Common Equity Tier 1 Risk Based Capital Required To Be Well Capitalized To Risk Weighted Assets Ratio	0.0650
Tier 1 Risk Based Capital Amount	$ 805,357
Tier 1 Risk Based Capital To Risk Weighted Assets Ratio	0.1384
Tier 1 Risk Based Capital Required For Capital Adequacy Amount	$ 349,052
Tier 1 Risk Based Capital Required For Capital Adequacy To Risk Weighted Assets Ratio	0.0600
Tier 1 Risk Based Capital With Capital Conservation Buffer Amount	$ 494,491
Tier 1 Risk Based Capital With Capital Conservation Buffer To Risk Weighted Assets Ratio	0.0850
Tier 1 Risk Based Capital Required To Be Well Capitalized Amount	$ 465,403
Tier 1 Risk Based Capital Required To Be Well Capitalized To Risk Weighted Assets Ratio	0.0800
Tier 1 Capital Amount	$ 805,357
Tier 1 Capital To Average Assets Ratio	0.0928
Tier 1 Capital Required For Capital Adequacy Amount	$ 347,274
Tier 1 Capital Required For Capital Adequacy To Average Assets Ratio	0.0400
Tier 1 Capital Required To Be Well Capitalized Amount	$ 434,092
Tier 1 Capital Required To Be Well Capitalized To Average Assets Ratio	0.0500
Pegasus Bank [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total Capital Amount	$ 68,927
Total Capital To Risk Weighted Assets Actual Ratio	0.1252
Capital Required For Capital Adequacy Amount	$ 44,046
Capital Required For Capital Adequacy to Risk Weighted Assets Ratio	0.0800
Capital With Capital Conservation Buffer Amount	$ 57,811
Capital With Capital Conservation To Risk Based Weighted Assets Buffer Ratio	0.1050
Capital Required To Be Well Capitalized Amount	$ 55,058
Capital Required To Be Well Capitalized To Risk Weighted Assets Ratio	0.1000
Common Equity Tier 1 Risk Based Capital Amount	$ 64,861
Common Equity Tier 1 Risk Based Capital To Risk Weighted Assets Ratio	0.1178
Common Equity Tier 1 Risk Based Capital Required For Capital Adequacy Amount	$ 24,776
Common Equity Tier 1 Risk Based Capital Required For Capital Adequacy To Risk Weighted Assets Ratio	0.0450
Common Equity Tier 1 Risk Based Capital With Capital Conservation Buffer Amount	$ 38,541
Common Equity Tier 1 Risk Based Capital With Capital Conservation Buffer To Risk Weighted Assets Ratio	0.0700
Common Equity Tier 1 Risk Based Capital Required To Be Well Capitalized Amount	$ 35,788
Common Equity Tier 1 Risk Based Capital Required To Be Well Capitalized To Risk Weighted Assets Ratio	0.0650
Tier 1 Risk Based Capital Amount	$ 64,861
Tier 1 Risk Based Capital To Risk Weighted Assets Ratio	0.1178
Tier 1 Risk Based Capital Required For Capital Adequacy Amount	$ 33,035
Tier 1 Risk Based Capital Required For Capital Adequacy To Risk Weighted Assets Ratio	0.0600
Tier 1 Risk Based Capital With Capital Conservation Buffer Amount	$ 46,799
Tier 1 Risk Based Capital With Capital Conservation Buffer To Risk Weighted Assets Ratio	0.0850
Tier 1 Risk Based Capital Required To Be Well Capitalized Amount	$ 44,046
Tier 1 Risk Based Capital Required To Be Well Capitalized To Risk Weighted Assets Ratio	0.0800
Tier 1 Capital Amount	$ 64,861
Tier 1 Capital To Average Assets Ratio	0.0865
Tier 1 Capital Required For Capital Adequacy Amount	$ 30,003
Tier 1 Capital Required For Capital Adequacy To Average Assets Ratio	0.0400
Tier 1 Capital Required To Be Well Capitalized Amount	$ 37,504
Tier 1 Capital Required To Be Well Capitalized To Average Assets Ratio	0.0500