Mergers and acquisitions - Franklin Financial Network, Inc. (Details) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2019 USD ($) branch $ / shares shares	Jan. 21, 2020 USD ($) $ / shares	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Business Acquisition [Line Items]
Assets assumed in acquisition	$ 6,124,921		$ 5,136,764	$ 4,727,713
Loans assumed in acquisition	4,409,642		3,667,511
Deposits assumed in acquisition	$ 4,934,938		$ 4,171,717
Franklin Financial Network, Inc.
Business Acquisition [Line Items]
Number of branches acquired | branch	15
Assets assumed in acquisition	$ 3,900,000
Loans assumed in acquisition	2,800,000
Deposits assumed in acquisition	$ 3,200,000
Common stock, conversion ratio | shares	0.9650
Common stock, cash received per share, value | $ / shares	$ 2.00
Subsequent Event | Franklin Financial Network, Inc.
Business Acquisition [Line Items]
Business acquisition, share price | $ / shares		$ 38.23
Transaction value		$ 602,000