Mergers and acquisitions - Schedule of Purchase Price (Details) $ / shares in Units, $ in Thousands		9 Months Ended
	Aug. 15, 2020 USD ($) $ / shares shares	Sep. 30, 2020 USD ($)
Business Combination, Consideration Transferred [Abstract]
Shares and net shares outstanding (in shares)	15,651,243
Franklin Financial Network, Inc.
Business Combination, Consideration Transferred [Abstract]
Shares outstanding (in shares)	15,588,337
Options converted to net shares (in shares)	62,906
Number of shares outstanding including converted options (in shares)	15,651,243
Exchange ratio	0.965
Shares to be issued as merger consideration (in shares)	15,102,492
Issuance price (dollar per share) | $ / shares	$ 29.52
Value of stock to be issued as merger consideration | $	$ 445,826
Tax withholding on vested restricted stock awards, units and options | $	(1,308)
Value of stock issued | $	$ 444,518
Net shares issued (in shares)	15,058,181
Stock units that do not vest on change in control (in shares)	114,915
Replacement awards issued to employees (in shares)	118,776
Fair value of replacement awards (in shares)	3,506,000
Fair value of replacement awards attributable to pre-combination service | $	$ 674
Cash consideration per share (dollar per share) | $ / shares	$ 2.00
Cash to be paid | $	$ 31,330
Purchase price | $	477,830
Preliminary fair value of net assets acquired | $	417,185
Preliminary goodwill resulting from merger | $	$ 60,645	$ 60,645
Equivalent value of parent stock (n shares)	44,311
Cash paid in lieu of fractional shares | $	$ 28