Stockholders' Equity - Required Capital Amounts and Company's Respective Ratios (Detail) $ in Thousands	Mar. 31, 2018 USD ($)
Parent Company [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total Capital Amount	$ 825,088
Total Capital To Risk Weighted Assets Actual Ratio	15.48%
Capital Required For Capital Adequacy Amount	$ 426,422
Capital Required For Capital Adequacy to Risk Weighted Assets Ratio	8.00%
Capital With Capital Conservation Buffer Amount	$ 526,365
Capital With Capital Conservation To Risk Based Weighted Assets Buffer Ratio	9.875%
Common Equity Tier 1 Risk Based Capital Amount	$ 742,538
Common Equity Tier 1 Risk Based Capital To Risk Weighted Assets Ratio	13.93%
Common Equity Tier 1 Risk Based Capital Required For Capital Adequacy Amount	$ 239,862
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	$ 339,805
Common Equity Tier 1 Risk Based Capital With Capital Conservation Buffer To Risk Weighted Assets Ratio	6.375%
Tier 1 Risk Based Capital Amount	$ 773,538
Tier 1 Risk Based Capital To Risk Weighted Assets Ratio	14.51%
Tier 1 Risk Based Capital Required For Capital Adequacy Amount	$ 319,816
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	$ 419,759
Tier 1 Risk Based Capital With Capital Conservation Buffer To Risk Weighted Assets Ratio	7.875%
Tier 1 Capital Amount	$ 773,538
Tier 1 Capital To Average Assets Ratio	10.43%
Tier 1 Capital Required For Capital Adequacy Amount	$ 296,653
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	$ 708,338
Total Capital To Risk Weighted Assets Actual Ratio	13.46%
Capital Required For Capital Adequacy Amount	$ 420,910
Capital Required For Capital Adequacy to Risk Weighted Assets Ratio	8.00%
Capital With Capital Conservation Buffer Amount	$ 519,560
Capital With Capital Conservation To Risk Based Weighted Assets Buffer Ratio	9.875%
Capital Required To Be Well Capitalized Amount	$ 526,137
Capital Required To Be Well Capitalized To Risk Weighted Assets Ratio	10.00%
Common Equity Tier 1 Risk Based Capital Amount	$ 636,775
Common Equity Tier 1 Risk Based Capital To Risk Weighted Assets Ratio	12.10%
Common Equity Tier 1 Risk Based Capital Required For Capital Adequacy Amount	$ 236,762
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	$ 335,412
Common Equity Tier 1 Risk Based Capital With Capital Conservation Buffer To Risk Weighted Assets Ratio	6.375%
Common Equity Tier 1 Risk Based Capital Required To Be Well Capitalized Amount	$ 341,989
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	$ 656,775
Tier 1 Risk Based Capital To Risk Weighted Assets Ratio	12.48%
Tier 1 Risk Based Capital Required For Capital Adequacy Amount	$ 315,682
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	$ 414,333
Tier 1 Risk Based Capital With Capital Conservation Buffer To Risk Weighted Assets Ratio	7.875%
Tier 1 Risk Based Capital Required To Be Well Capitalized Amount	$ 420,910
Tier 1 Risk Based Capital Required To Be Well Capitalized To Risk Weighted Assets Ratio	8.00%
Tier 1 Capital Amount	$ 656,775
Tier 1 Capital To Average Assets Ratio	8.96%
Tier 1 Capital Required For Capital Adequacy Amount	$ 293,061
Tier 1 Capital Required For Capital Adequacy To Average Assets Ratio	4.00%
Tier 1 Capital Required To Be Well Capitalized Amount	$ 366,327
Tier 1 Capital Required To Be Well Capitalized To Average Assets Ratio	5.00%