Mergers and acquisitions - Consideration for Franklin Financial Network (Details) $ / shares in Units, $ in Thousands		12 Months Ended
	Aug. 15, 2020 USD ($) $ / shares shares	Dec. 31, 2020 USD ($)
Business Combination, Consideration Transferred [Abstract]
Shares and net shares outstanding (in shares)	15,651,243
Franklin
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 (in dollars per share) | $ / shares	$ 29.52
Value of stock to be issued as merger consideration | $	$ 445,826
Less: 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 replacement awards attributable to pre-combination service | $	$ 674
Cash consideration per share (in dollars per share) | $ / shares	$ 2.00
Total cash to be paid | $	$ 31,330
Total purchase price | $	477,830
Preliminary fair value of net assets acquired | $	410,639
Goodwill resulting from merger | $	$ 67,191	$ 67,191
Equivalent value of parent stock (in shares)	44,311
Cash paid in lieu of fractional shares | $	$ 28