Mergers and acquisitions - Franklin Financial Network, Inc. Narrative (Details) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2020 USD ($) branch $ / shares shares	Jan. 21, 2020 USD ($) $ / shares	Jan. 01, 2020 USD ($)	Dec. 31, 2019 USD ($)	Mar. 31, 2019 USD ($)
Business Acquisition [Line Items]
Assets assumed in acquisition	$ 6,655,687			$ 6,124,921	$ 5,335,156
Loans assumed in acquisition	4,568,038		$ 4,409,642	4,409,642
Deposits assumed in acquisition	$ 5,376,932			$ 4,934,938
Franklin Financial Network, Inc.
Business Acquisition [Line Items]
Number of branches acquired | branch	15
Assets assumed in acquisition	$ 3,790,000
Loans assumed in acquisition	2,860,000
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		$ 38.23
Total purchase price		$ 602,000