Stockholders' Equity - Required Capital Amounts and Company's Respective Ratios (Detail) $ in Thousands	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Parent Company [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total Capital Amount	$ 997,383	$ 910,561
Total Capital To Risk Weighted Assets Actual Ratio	15.64	14.42
Capital Required For Capital Adequacy Amount	$ 510,110	$ 505,050
Capital Required For Capital Adequacy to Risk Weighted Assets Ratio	8.00	8.00
Capital With Capital Conservation Buffer Amount	$ 669,519	$ 662,878
Capital With Capital Conservation To Risk Based Weighted Assets Buffer Ratio	10.50	10.50
Common Equity Tier 1 Risk Based Capital Amount	$ 891,534	$ 830,323
Common Equity Tier 1 Risk Based Capital To Risk Weighted Assets Ratio	13.98	13.15
Common Equity Tier 1 Risk Based Capital Required For Capital Adequacy Amount	$ 286,937	$ 284,091
Common Equity Tier 1 Risk Based Capital Required For Capital Adequacy To Risk Weighted Assets Ratio	4.50	4.50
Common Equity Tier 1 Risk Based Capital With Capital Conservation Buffer Amount	$ 446,346	$ 441,919
Common Equity Tier 1 Risk Based Capital With Capital Conservation Buffer To Risk Weighted Assets Ratio	7.00	7.00
Tier 1 Risk Based Capital Amount	$ 917,534	$ 856,323
Tier 1 Risk Based Capital To Risk Weighted Assets Ratio	14.39	13.56
Tier 1 Risk Based Capital Required For Capital Adequacy Amount	$ 382,583	$ 378,788
Tier 1 Risk Based Capital Required For Capital Adequacy To Risk Weighted Assets Ratio	6.00	6.00
Tier 1 Risk Based Capital With Capital Conservation Buffer Amount	$ 541,992	$ 536,616
Tier 1 Risk Based Capital With Capital Conservation Buffer To Risk Weighted Assets Ratio	8.50	8.50
Tier 1 Capital Amount	$ 917,534	$ 856,323
Tier 1 Capital To Average Assets Ratio	9.63	10.28
Tier 1 Capital Required For Capital Adequacy Amount	$ 380,952	$ 333,171
Tier 1 Capital Required For Capital Adequacy To Average Assets Ratio	4.00	4.00
BancFirst [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total Capital Amount	$ 898,714	$ 827,928
Total Capital To Risk Weighted Assets Actual Ratio	15.59	14.14
Capital Required For Capital Adequacy Amount	$ 461,142	$ 468,107
Capital Required For Capital Adequacy to Risk Weighted Assets Ratio	8.00	8.00
Capital With Capital Conservation Buffer Amount	$ 605,248	$ 614,390
Capital With Capital Conservation To Risk Based Weighted Assets Buffer Ratio	10.50	10.50
Capital Required To Be Well Capitalized Amount	$ 576,427	$ 585,134
Capital Required To Be Well Capitalized To Risk Weighted Assets Ratio	10.00	10.00
Common Equity Tier 1 Risk Based Capital Amount	$ 806,478	$ 753,659
Common Equity Tier 1 Risk Based Capital To Risk Weighted Assets Ratio	13.99	12.88
Common Equity Tier 1 Risk Based Capital Required For Capital Adequacy Amount	$ 259,392	$ 263,310
Common Equity Tier 1 Risk Based Capital Required For Capital Adequacy To Risk Weighted Assets Ratio	4.50	4.50
Common Equity Tier 1 Risk Based Capital With Capital Conservation Buffer Amount	$ 403,499	$ 409,594
Common Equity Tier 1 Risk Based Capital With Capital Conservation Buffer To Risk Weighted Assets Ratio	7.00	7.00
Common Equity Tier 1 Risk Based Capital Required To Be Well Capitalized Amount	$ 374,678	$ 380,337
Common Equity Tier 1 Risk Based Capital Required To Be Well Capitalized To Risk Weighted Assets Ratio	6.50	6.50
Tier 1 Risk Based Capital Amount	$ 826,478	$ 773,659
Tier 1 Risk Based Capital To Risk Weighted Assets Ratio	14.34	13.22
Tier 1 Risk Based Capital Required For Capital Adequacy Amount	$ 345,856	$ 351,080
Tier 1 Risk Based Capital Required For Capital Adequacy To Risk Weighted Assets Ratio	6.00	6.00
Tier 1 Risk Based Capital With Capital Conservation Buffer Amount	$ 489,963	$ 497,364
Tier 1 Risk Based Capital With Capital Conservation Buffer To Risk Weighted Assets Ratio	8.50	8.50
Tier 1 Risk Based Capital Required To Be Well Capitalized Amount	$ 461,142	$ 468,107
Tier 1 Risk Based Capital Required To Be Well Capitalized To Risk Weighted Assets Ratio	8.00	8.00
Tier 1 Capital Amount	$ 826,478	$ 773,659
Tier 1 Capital To Average Assets Ratio	9.48	10.22
Tier 1 Capital Required For Capital Adequacy Amount	$ 348,666	$ 302,845
Tier 1 Capital Required For Capital Adequacy To Average Assets Ratio	4.00	4.00
Tier 1 Capital Required To Be Well Capitalized Amount	$ 435,833	$ 378,556
Tier 1 Capital Required To Be Well Capitalized To Average Assets Ratio	5.00	5.00
Pegasus Bank [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total Capital Amount	$ 70,922	$ 63,817
Total Capital To Risk Weighted Assets Actual Ratio	11.85	14.15
Capital Required For Capital Adequacy Amount	$ 47,893	$ 36,083
Capital Required For Capital Adequacy to Risk Weighted Assets Ratio	8.00	8.00
Capital With Capital Conservation Buffer Amount	$ 62,860	$ 47,359
Capital With Capital Conservation To Risk Based Weighted Assets Buffer Ratio	10.50	10.50
Capital Required To Be Well Capitalized Amount	$ 59,866	$ 45,104
Capital Required To Be Well Capitalized To Risk Weighted Assets Ratio	10.00	10.00
Common Equity Tier 1 Risk Based Capital Amount	$ 66,150	$ 60,214
Common Equity Tier 1 Risk Based Capital To Risk Weighted Assets Ratio	11.05	13.35
Common Equity Tier 1 Risk Based Capital Required For Capital Adequacy Amount	$ 26,940	$ 20,297
Common Equity Tier 1 Risk Based Capital Required For Capital Adequacy To Risk Weighted Assets Ratio	4.50	4.50
Common Equity Tier 1 Risk Based Capital With Capital Conservation Buffer Amount	$ 41,906	$ 31,573
Common Equity Tier 1 Risk Based Capital With Capital Conservation Buffer To Risk Weighted Assets Ratio	7.00	7.00
Common Equity Tier 1 Risk Based Capital Required To Be Well Capitalized Amount	$ 38,913	$ 29,317
Common Equity Tier 1 Risk Based Capital Required To Be Well Capitalized To Risk Weighted Assets Ratio	6.50	6.50
Tier 1 Risk Based Capital Amount	$ 66,150	$ 60,214
Tier 1 Risk Based Capital To Risk Weighted Assets Ratio	11.05	13.35
Tier 1 Risk Based Capital Required For Capital Adequacy Amount	$ 35,920	$ 27,062
Tier 1 Risk Based Capital Required For Capital Adequacy To Risk Weighted Assets Ratio	6.00	6.00
Tier 1 Risk Based Capital With Capital Conservation Buffer Amount	$ 50,886	$ 38,338
Tier 1 Risk Based Capital With Capital Conservation Buffer To Risk Weighted Assets Ratio	8.50	8.50
Tier 1 Risk Based Capital Required To Be Well Capitalized Amount	$ 47,893	$ 36,083
Tier 1 Risk Based Capital Required To Be Well Capitalized To Risk Weighted Assets Ratio	8.00	8.00
Tier 1 Capital Amount	$ 66,150	$ 60,214
Tier 1 Capital To Average Assets Ratio	8.10	8.58
Tier 1 Capital Required For Capital Adequacy Amount	$ 32,682	$ 28,058
Tier 1 Capital Required For Capital Adequacy To Average Assets Ratio	4.00	4.00
Tier 1 Capital Required To Be Well Capitalized Amount	$ 40,852	$ 35,072
Tier 1 Capital Required To Be Well Capitalized To Average Assets Ratio	5.00	5.00