Commitments and contingencies - Additional Information (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Commitments and Contingencies Disclosure [Abstract]
Total principal amount of loans repurchased or indemnified	$ 6,475	$ 6,646	$ 4,704

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