Short-term Borrowings - Additional Information (Details) - USD ($)	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Short Term Debt [Line Items]
Borrowings against lines	$ 190,000,000	$ 150,000,000
Federal Reserve Bank \| Borrower in Custody Program
Short Term Debt [Line Items]
Line of credit	529,547,000	765,107,000
Federal Reserve Bank \| Borrower in Custody Program \| Qualifying Loans
Short Term Debt [Line Items]
Pledged loan securities	737,856,000	1,072,118,000
Federal Reserve Bank \| Borrower in Custody Program \| Investment Securities
Short Term Debt [Line Items]
Pledged loan securities	13,544,000	0
Federal Home Loan Bank
Short Term Debt [Line Items]
Borrowings against lines	190,000,000	150,000,000
Pledged investment securities	0	60,371,000
Loans from Federal Home Loan Banks	968,567,000	565,718,000
U.S. Government Agency Securities \| One to Four Family and Multi-Family Mortgages \| Federal Home Loan Bank
Short Term Debt [Line Items]
Line of credit	300,000,000	300,000,000
First Tennessee Bank, N.A
Short Term Debt [Line Items]
Line of credit	$ 30,000,000	30,000,000
Number of consecutive trading days	14 days
Borrowings against lines	$ 0	0
BBVA Compass Bank
Short Term Debt [Line Items]
Line of credit	$ 10,000,000	10,000,000
Number of consecutive trading days	14 days
Borrowings against lines	$ 0	0
SunTrust Bank
Short Term Debt [Line Items]
Line of credit	$ 15,000,000
Number of consecutive trading days	7 days
Borrowings against lines	$ 0	0
SunTrust Bank \| Minimum
Short Term Debt [Line Items]
Marketable security with current value, percent	125.00%
First National Banker's Bank
Short Term Debt [Line Items]
Line of credit	$ 10,000,000
Number of consecutive trading days	30 days
Zions Bank
Short Term Debt [Line Items]
Line of credit	$ 25,000,000
PNC Bank
Short Term Debt [Line Items]
Line of credit	20,000,000
Servis First Bank
Short Term Debt [Line Items]
Line of credit	15,000,000
Borrowings against lines	$ 0	$ 0

Marketable security with current value percent.

No definition available.

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

- Definition

Number of consecutive trading days.

+ References

No definition available.

+ Details

Name:	fbk_NumberOfConsecutiveTradingDays
Namespace Prefix:	fbk_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of short term borrowing where a bank borrows, at the federal funds rate, from another bank.

+ References

+ Details

Name:	us-gaap_FederalFundsPurchased
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The carrying value as of the balance sheet date of the current portion of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(13))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(19))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_LinesOfCreditCurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The fair value, as of the date of each statement of financial position presented, of loans that are restricted as collateral for the payment of the FHLB debt obligations, and that are reclassified and separately reported in the statement of financial position because the transferee has the right by contract or custom to sell or re-pledge them.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=66007379&loc=d3e113888-111728

+ Details

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

- Definition

The fair value, as of the date of each statement of financial position presented, of securities which are owned but transferred to serve as collateral for the payment of the FHLB debt obligations, and that are reclassified and separately reported in the statement of financial position because the transferee has the right by contract or custom to sell or re-pledge them.

+ References

+ Details

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

- Definition

Reflects the total carrying amount as of the balance sheet date of debt having initial terms less than one year or the normal operating cycle, if longer.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(19)(a))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(13))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(16)(a)(1))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

+ Details

Name:	us-gaap_ShortTermBorrowings
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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_ShortTermDebtLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration