Mergers and acquisitions - Franklin Financial Network, Inc. Narrative (Details) $ / shares in Units, $ in Thousands		3 Months Ended		9 Months Ended
	Aug. 15, 2020 USD ($) branch $ / shares shares	Sep. 30, 2020 USD ($) branch	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) branch	Sep. 30, 2019 USD ($)
Business Acquisition [Line Items]
Merger costs		$ 20,730	$ 295	$ 25,366	$ 4,699
Franklin Financial Network, Inc.
Business Acquisition [Line Items]
Number of branches acquired | branch	15
Assets assumed in acquisition	$ 3,624,830
Loans assumed in acquisition	2,794,569
Deposits assumed in acquisition	3,107,005
Non-strategic loans assumed in acquisition	$ 318,365
Business acquisition, shares issued (in shares) | shares	15,058,181
Equivalent value of parent stock (n shares) | shares	44,311
Cash consideration paid	$ 31,330
Business acquisition, stock assumed, pre combination service, fair value	$ 674
Business acquisition, share price (dollar per share) | $ / shares	$ 29.52
Business combination, consideration, value	$ 477,830
Addition from acquisition/merger	$ 60,645			60,645
Merger costs		$ 20,489		$ 22,896
Franklin Financial Network, Inc. | Tennessee
Business Acquisition [Line Items]
Number of branches closed and consolidated | branch		6		6