Mergers and acquisitions - Franklin Financial Network, Inc. Narrative (Details) $ / shares in Units, $ in Thousands	6 Months Ended
	Jun. 30, 2020 USD ($) branch $ / shares shares	Jan. 01, 2020 USD ($)	Dec. 31, 2019 USD ($)	Jun. 30, 2019 USD ($)
Business Acquisition [Line Items]
Assets assumed in acquisition	$ 7,255,536		$ 6,124,921	$ 5,940,402
Loans assumed in acquisition	4,827,023	$ 4,409,642	4,409,642
Deposits assumed in acquisition	$ 5,952,801		$ 4,934,938
Franklin Financial Network, Inc.
Business Acquisition [Line Items]
Number of branches acquired | branch	15
Assets assumed in acquisition	$ 3,780,000
Loans assumed in acquisition	2,790,000
Deposits assumed in acquisition	$ 3,140,000
Common stock, conversion ratio | shares	0.9650
Common stock, cash received per share, value | $ / shares	$ 2.00
Business acquisition, share price (dollar per share) | $ / shares	$ 24.77
Total purchase price	$ 394,000