Income Taxes - Reconciliation of Income Taxes Computed at the United States Federal Statutory Tax Rates to the Provision for Income Taxes (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Income Tax Disclosure [Abstract]
Federal taxes calculated at statutory rate, amount	$ 5,475	$ 11,916	$ 14,830	$ 26,293
Increase (decrease) resulting from:
State taxes, net of federal benefit	1,582	1,879	2,533	3,629
Benefit from equity based compensation	(15)	(124)	(306)	(345)
Municipal interest income, net of interest disallowance	(444)	(419)	(888)	(843)
Bank-owned life insurance	(79)	(82)	(153)	(166)
Offering costs	0	127	0	127
Section 162(m) limitation	40	21	162	248
Other	158	121	(148)	85
Total	$ 6,717	$ 13,440	$ 16,030	$ 29,028
Federal taxes calculated at statutory rate, percent	21.00%	21.00%	21.00%	21.00%
Percentage increase (decrease) resulting from:
State taxes, net of federal benefit	6.10%	3.30%	3.60%	2.90%
Benefit from equity based compensation	(0.10%)	(0.20%)	(0.40%)	(0.30%)
Municipal interest income, net of interest disallowance	(1.70%)	(0.70%)	(1.30%)	(0.70%)
Bank-owned life insurance	(0.30%)	(0.10%)	(0.20%)	(0.10%)
Offering costs	0.00%	0.20%	0.00%	0.10%
Section 162(m) limitation	0.20%	0.00%	0.10%	0.20%
Other	0.60%	0.20%	(0.10%)	0.10%
Total	25.80%	23.70%	22.70%	23.20%

Effective Income Tax Rate Reconciliation, Bank Owned Life Insurance, Percent

No definition available.

Name:	fbk_EffectiveIncomeTaxRateReconciliationBankOwnedLifeInsurancePercent
Namespace Prefix:	fbk_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Effective Income Tax Rate Reconciliation Municipal Interest Income Net Of Interest Disallowance, Amount

+ References

No definition available.

+ Details

Name:	fbk_EffectiveIncomeTaxRateReconciliationMunicipalInterestIncomeNetOfInterestDisallowanceAmount
Namespace Prefix:	fbk_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Effective income tax rate reconciliation, municipal interest income net of interest disallowance, percent.

+ References

No definition available.

+ Details

Name:	fbk_EffectiveIncomeTaxRateReconciliationMunicipalInterestIncomeNetOfInterestDisallowancePercent
Namespace Prefix:	fbk_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Effective Income Tax Rate Reconciliation Nondeductible Expense Merger And Offering Costs

+ References

No definition available.

+ Details

Name:	fbk_EffectiveIncomeTaxRateReconciliationNondeductibleExpenseMergerAndOfferingCosts
Namespace Prefix:	fbk_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Income tax reconciliation bank owned life insurance.

+ References

No definition available.

+ Details

Name:	fbk_IncomeTaxReconciliationBankOwnedLifeInsurance
Namespace Prefix:	fbk_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Income Tax Reconciliation Change In Merger And Offering Costs

+ References

No definition available.

+ Details

Name:	fbk_IncomeTaxReconciliationChangeInMergerAndOfferingCosts
Namespace Prefix:	fbk_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Income tax reconciliation change in state taxes net of federal benefit.

+ References

No definition available.

+ Details

Name:	fbk_IncomeTaxReconciliationChangeInStateTaxesNetOfFederalBenefit
Namespace Prefix:	fbk_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Income tax reconciliation nondeductible income share based compensation cost.

+ References

No definition available.

+ Details

Name:	fbk_IncomeTaxReconciliationNondeductibleIncomeShareBasedCompensationCost
Namespace Prefix:	fbk_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Income Tax Reconciliation Nondeductible Income Share Based Compensation Cost, Percent

+ References

No definition available.

+ Details

Name:	fbk_IncomeTaxReconciliationNondeductibleIncomeShareBasedCompensationCostPercent
Namespace Prefix:	fbk_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Percentage of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 12
-URI https://asc.fasb.org/extlink&oid=121826272&loc=d3e32687-109319

+ Details

Name:	us-gaap_EffectiveIncomeTaxRateContinuingOperations
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- References

No definition available.

+ Details

Name:	us-gaap_EffectiveIncomeTaxRateContinuingOperationsTaxRateReconciliationAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Percentage of domestic federal statutory tax rate applicable to pretax income (loss).

+ References

+ Details

Name:	us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Percentage of difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying domestic federal statutory income tax rate to pretax income (loss) from continuing operation, attributable to nondeductible expense for share-based payment arrangement.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 12
-URI https://asc.fasb.org/extlink&oid=121826272&loc=d3e32687-109319

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 6.I.Fact.4)
-URI https://asc.fasb.org/extlink&oid=122134291&loc=d3e330036-122817

+ Details

Name:	us-gaap_EffectiveIncomeTaxRateReconciliationNondeductibleExpenseShareBasedCompensationCost
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Percentage of the difference, between reported income tax expense (benefit) and the expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations, that is attributable to tax exempt income, equity in earnings (loss) of an unconsolidated subsidiary, minority interest income (expense), tax holiday, disposition of a business, disposition of an asset, repatriation of foreign earnings, repatriation of foreign earnings jobs creation act of 2004, change in enacted tax rate, prior year income taxes, change in deferred tax asset valuation allowance, and other adjustments.

+ References

No definition available.

+ Details

Name:	us-gaap_EffectiveIncomeTaxRateReconciliationOtherReconcilingItemsPercent
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Percentage of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations applicable to state and local income tax expense (benefit), net of federal tax expense (benefit).

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 6.I.Fact.4)
-URI https://asc.fasb.org/extlink&oid=122134291&loc=d3e330036-122817

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 12
-URI https://asc.fasb.org/extlink&oid=121826272&loc=d3e32687-109319

+ Details

Name:	us-gaap_EffectiveIncomeTaxRateReconciliationStateAndLocalIncomeTaxes
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- References

No definition available.

+ Details

Name:	us-gaap_IncomeTaxDisclosureAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SAB TOPIC 6.I.7)
-URI https://asc.fasb.org/extlink&oid=122134291&loc=d3e330036-122817

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 10
-URI https://asc.fasb.org/extlink&oid=121826272&loc=d3e32672-109319

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 22
-Subparagraph (h)
-URI https://asc.fasb.org/extlink&oid=126901519&loc=d3e8736-108599

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08(h))
-URI https://asc.fasb.org/extlink&oid=120395691&loc=d3e23780-122690

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 250
-SubTopic 10
-Section 50
-Paragraph 9
-URI https://asc.fasb.org/extlink&oid=124431687&loc=d3e22663-107794

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 20
-Section 45
-Paragraph 2
-Subparagraph (a)
-URI https://asc.fasb.org/extlink&oid=123586238&loc=d3e38679-109324

Reference 7: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 220
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-04(9))
-URI https://asc.fasb.org/extlink&oid=120400993&loc=SL114874131-224263

Reference 8: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 250
-SubTopic 10
-Section 50
-Paragraph 8
-URI https://asc.fasb.org/extlink&oid=124431687&loc=d3e22658-107794

+ Details

Name:	us-gaap_IncomeTaxExpenseBenefit
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- References

No definition available.

+ Details

Name:	us-gaap_IncomeTaxExpenseBenefitContinuingOperationsIncomeTaxReconciliationAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The amount of income tax expense or benefit for the period computed by applying the domestic federal statutory tax rates to pretax income from continuing operations.

+ References

+ Details

Name:	us-gaap_IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of reported income tax expense (benefit) in excess of (less than) expected income tax expense (benefit) computed by applying domestic federal statutory income tax rate to pretax income (loss) from continuing operation, attributable to nondeductible expense for award under share-based payment arrangement. Includes, but is not limited to, expense determined to be nondeductible upon grant or after for award under share-based payment arrangement.

+ References

+ Details

Name:	us-gaap_IncomeTaxReconciliationNondeductibleExpenseShareBasedCompensationCost
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations attributable to tax exempt income, equity in earnings (loss) of an unconsolidated subsidiary, minority noncontrolling interest income (loss), tax holiday, disposition of a business, disposition of an asset, repatriation of foreign earnings, repatriation of foreign earnings jobs creation act of 2004, increase (decrease) in enacted tax rate, prior year income taxes, increase (decrease) in deferred tax asset valuation allowance, and other adjustments.

+ References

+ Details

Name:	us-gaap_IncomeTaxReconciliationOtherReconcilingItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration