Employee benefit plans - Salaries and Employee Benefits Expense - Profit Sharing (Details) - Retirement Profit Sharing Plan - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Profit Sharing plan
Company contribution	$ 1.4	$ 1.3	$ 1.2
Safe harbor contribution	0.8	0.7	0.7
Discretionary contributions	$ 0.6	$ 0.6	$ 0.6