Loans and allowance for loan losses - Financial Effect of TDRs (Details) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2018 USD ($) loan	Jun. 30, 2019 USD ($) loan	Jun. 30, 2018 USD ($) loan
Financing Receivable, Modifications [Line Items]
Number of loans \| loan	2	2	4
Pre-modification outstanding recorded investment	$ 887	$ 3,188	$ 1,141
Post-modification outstanding recorded investment	887	3,188	1,141
Charge offs and specific reserves	$ 0	$ 0	$ 0
Commercial and industrial
Financing Receivable, Modifications [Line Items]
Number of loans \| loan	2	2	2
Pre-modification outstanding recorded investment	$ 887	$ 3,188	$ 887
Post-modification outstanding recorded investment	887	3,188	887
Charge offs and specific reserves	$ 0	$ 0	$ 0
Residential real estate: \| 1-to-4 family mortgage
Financing Receivable, Modifications [Line Items]
Number of loans \| loan			1
Pre-modification outstanding recorded investment			$ 249
Post-modification outstanding recorded investment			249
Charge offs and specific reserves			$ 0
Residential real estate: \| Consumer and other
Financing Receivable, Modifications [Line Items]
Number of loans \| loan			1
Pre-modification outstanding recorded investment			$ 5
Post-modification outstanding recorded investment			5
Charge offs and specific reserves			$ 0

Financing receivable modifications charge offs and specific reserves.

No definition available.

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

- Definition

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.

+ References

No definition available.

+ Details

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

- Definition

Number of financing receivables that have been modified by troubled debt restructurings.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 33
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953676-111524

+ Details

Name:	us-gaap_FinancingReceivableModificationsNumberOfContracts2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Post-modification amount of investment in financing receivables modified by troubled debt restructurings.

+ References

+ Details

Name:	us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Pre-modification amount of investment in financing receivables modified by troubled debt restructurings.

+ References

+ Details

Name:	us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

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