Mergers and acquisitions - Atlantic Capital Bank Narrative (Details) - Atlantic Capital Bank $ in Thousands	Apr. 05, 2019 USD ($) branch
Business Acquisition [Line Items]
Total purchase price	$ 36,790
Customer deposits	$ 588,877
Deposit premium percentage	6.25%
Loans held for investment, net of fair value adjustments	$ 374,966
Percentage of loans outstanding	99.32%
Tennessee
Business Acquisition [Line Items]
Number of branches acquired \| branch	11
Georgia
Business Acquisition [Line Items]
Number of branches acquired \| branch	3

Business Acquisition, Transaction Value

No definition available.

Name:	fbk_BusinessAcquisitionTransactionValue
Namespace Prefix:	fbk_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Customer Deposits

+ References

No definition available.

+ Details

Name:	fbk_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCustomerDeposits
Namespace Prefix:	fbk_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Percentage of Loans Outstanding

+ References

No definition available.

+ Details

Name:	fbk_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedPercentageOfLoansOutstanding
Namespace Prefix:	fbk_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Deposit Premium, Percentage

+ References

No definition available.

+ Details

Name:	fbk_BusinessCombinationRecognizedIdentifiableAssetsAcquiredandLiabilitiesAssumedDepositPremiumPercentage
Namespace Prefix:	fbk_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Number Of Branches Acquired

+ References

No definition available.

+ Details

Name:	fbk_NumberOfBranchesAcquired
Namespace Prefix:	fbk_
Data Type:	xbrli:integerItemType
Balance Type:	na
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_BusinessAcquisitionLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The amount of financial assets (as defined) recognized as of the acquisition date.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-URI http://asc.fasb.org/topic&trid=2134543

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 10
-Section 55
-Paragraph 37
-URI http://asc.fasb.org/extlink&oid=121600890&loc=d3e2207-128464

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=121647850&loc=d3e4845-128472

+ Details

Name:	us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedFinancialAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	us-gaap_BusinessAcquisitionAxis=fbk_AtlanticCapitalBankMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementGeographicalAxis=stpr_TN
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementGeographicalAxis=stpr_GA
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: