Fair value of financial instruments - Changes in Fair Value Associated with Commercial Loans Held for Sale (Details) - Franklin Financial Network, Inc. - Commercial and industrial - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
	Jun. 30, 2021	Jun. 30, 2021
Carrying amount
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Carrying value at beginning of period	$ 197,490	$ 239,063
Pay-downs and pay-offs	(52,226)	(91,792)
Write-offs to discount	(9,292)	(11,299)
Changes in valuation included in other noninterest income	0	0
Carrying value at end of period	135,972	135,972
Fair Value Discount
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Carrying value at beginning of period	(22,506)	(23,660)
Pay-downs and pay-offs	0	0
Write-offs to discount	9,292	11,299
Changes in valuation included in other noninterest income	1,364	511
Carrying value at end of period	(11,850)	(11,850)
Fair Value
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Carrying value at beginning of period	174,984	215,403
Pay-downs and pay-offs	(52,226)	(91,792)
Write-offs to discount	0	0
Changes in valuation included in other noninterest income	1,364	511
Carrying value at end of period	$ 124,122	$ 124,122