Commitments and contingencies - Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Commitments and Contingencies Disclosure [Abstract]
Total principal amount of loans repurchased or indemnified	$ 1,165	$ 2,322	$ 4,675	$ 4,984

Principal amount of loans repurchased or indemnified during period.

No definition available.

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