Mergers and acquisitions - Franklin Financial Network, Inc. - Narrative (Details) - Franklin $ / shares in Units, $ in Thousands		12 Months Ended
	Aug. 15, 2020 USD ($) branch $ / shares shares	Dec. 31, 2020 USD ($)
Business Acquisition [Line Items]
Assets assumed in acquisition	$ 3,631,889
Loans assumed in acquisition	2,790,000
Deposits assumed in acquisition	3,121,731
Non-strategic loans assumed in acquisition	$ 326,206
Business acquisition, shares issued (in shares) | shares	15,058,181
Equivalent value of parent stock (in shares) | shares	44,311
Cash consideration paid	$ 31,330
Fair value replacement awards attributable to pre-combination service	$ 674
Business acquisition, share price (in dollars per share) | $ / shares	$ 29.52
Purchase price	$ 477,830
Goodwill resulting from merger	$ 67,191	$ 67,191
Tennessee
Business Acquisition [Line Items]
Number of branches acquired | branch	10