Investment securities - Pledged Securities (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Available-for-sale and Held-to-maturity securities
Carrying value of securities pledged	Assets pledged as collateral	Assets pledged as collateral
Federal Reserve Bank Advances [Member]
Available-for-sale and Held-to-maturity securities
Carrying value of securities pledged	$ 191.0
Collateral Pledged
Available-for-sale and Held-to-maturity securities
Carrying value of securities pledged	$ 322.4	$ 168.0
U.S. government-sponsored enterprises
Available-for-sale and Held-to-maturity securities
Maximum percentage of stockholders' equity exceeded for securities of any individual issuer	10.00%	10.00%