Leases (Details)	Mar. 31, 2020 lease lease_renew_option
Lessee, Lease, Description [Line Items]
Lessee, number of operating leases, noncurrent \| lease	35
Lessee, operating lease, number of options to renew \| lease_renew_option	1
Minimum
Lessee, Lease, Description [Line Items]
Lessee, operating lease, term of contract	1 year
Lessee, operating lease, renewal term	20 years
Maximum
Lessee, Lease, Description [Line Items]
Lessee, operating lease, term of contract	36 years

Lessee, Number of Operating Leases, Noncurrent

No definition available.

Name:	fbk_LesseeNumberofOperatingLeasesNoncurrent
Namespace Prefix:	fbk_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Lessee, Operating Lease, Number Of Options To Renew

+ References

No definition available.

+ Details

Name:	fbk_LesseeOperatingLeaseNumberOfOptionsToRenew
Namespace Prefix:	fbk_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

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

- Definition

Term of lessee's operating lease renewal, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 842
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (a)(3)
-URI http://asc.fasb.org/extlink&oid=77888426&loc=SL77918673-209980

+ Details

Name:	us-gaap_LesseeOperatingLeaseRenewalTerm
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	instant

- Definition

Term of lessee's operating lease, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

+ References

+ Details

Name:	us-gaap_LesseeOperatingLeaseTermOfContract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	instant

- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
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Balance Type:
Period Type: