Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 55
-Paragraph 12A
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481933/310-10-55-12A

Reference 2: http://fasb.org/us-gaap/role/ref/otherTransitionRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 34
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481962/310-10-50-34

Reference 3: http://fasb.org/us-gaap/role/ref/otherTransitionRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 33
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481962/310-10-50-33

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 42
-Subparagraph (a)(2)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481962/310-10-50-42

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 42
-Subparagraph (a)(1)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481962/310-10-50-42

+ Details

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

- Definition

Percentage of decrease in weighted average contractual interest rate of financing receivable modified for debtor experiencing financial difficulty.

+ References

+ Details

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

- Definition

Period of increase in weighted average maturity of financing receivable modified for debtor experiencing financial difficulty, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

+ Details

Name:	us-gaap_FinancingReceivableModifiedWeightedAverageTermIncreaseFromModification
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_CommercialPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LoanRestructuringModificationAxis=us-gaap_ExtendedMaturityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LoanRestructuringModificationAxis=us-gaap_PaymentDeferralMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=fbk_ConstructionPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_ResidentialPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=fbk_OneToFourFamilyMortgagesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_CommercialRealEstatePortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=fbk_NonOwnerOccupiedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=fbk_ConsumerAndOtherPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Loans and Allowance for Credit Losses on Loans HFI - Schedule of Financial Effects of Loan Modifications (Details)	3 Months Ended		6 Months Ended
	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
Commercial and industrial \| Weighted average term extension (in months)
Financing Receivable, Troubled Debt Restructuring [Line Items]
Weighted average of loans (in months)	4 months		23 months
Commercial and industrial \| Weighted average payment deferral (in months)
Financing Receivable, Troubled Debt Restructuring [Line Items]
Weighted average of loans (in months)	4 months		4 months
Weighted average interest rate reduction			0.00%
Construction \| Weighted average term extension (in months)
Financing Receivable, Troubled Debt Restructuring [Line Items]
Weighted average of loans (in months)	4 months		4 months	6 months
Construction \| Weighted average payment deferral (in months)
Financing Receivable, Troubled Debt Restructuring [Line Items]
Weighted average of loans (in months)	4 months		4 months	3 months
Weighted average interest rate reduction			2.50%	0.00%
Residential real estate: \| Weighted average term extension (in months) \| 1-to-4 family mortgage
Financing Receivable, Troubled Debt Restructuring [Line Items]
Weighted average of loans (in months)	300 months		300 months
Residential real estate: \| Weighted average payment deferral (in months) \| 1-to-4 family mortgage
Financing Receivable, Troubled Debt Restructuring [Line Items]
Weighted average of loans (in months)	4 months		4 months
Weighted average interest rate reduction			0.00%
Commercial real estate: \| Weighted average term extension (in months) \| Non-owner occupied
Financing Receivable, Troubled Debt Restructuring [Line Items]
Weighted average of loans (in months)				6 months
Commercial real estate: \| Weighted average payment deferral (in months) \| Non-owner occupied
Financing Receivable, Troubled Debt Restructuring [Line Items]
Weighted average interest rate reduction				0.00%
Consumer and other \| Weighted average term extension (in months)
Financing Receivable, Troubled Debt Restructuring [Line Items]
Weighted average of loans (in months)		21 months	13 months	25 months
Consumer and other \| Weighted average payment deferral (in months)
Financing Receivable, Troubled Debt Restructuring [Line Items]
Weighted average interest rate reduction		1.49%	2.00%	1.49%