Stockholders' equity - Preferred stock - Additional Information (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Stockholders Equity Note
Preferred stock shares authorized	30,000,000	30,000,000
Preferred stock shares outstanding	885,726	885,726	885,726	885,726
2003 Series A Preferred Stock
Stockholders Equity Note
Preferred stock shares outstanding	885,726	885,726	885,726
Preferred Stock | 2003 Series A Preferred Stock
Stockholders Equity Note
Preferred Stock Redemption Price Per Share	$ 25.00
Preferred stock liquidation preference	25
Preferred stock dividends per share cash paid	$ 0.1328125
Preferred stock dividend rate percentage	6.375%
Dividend preferred stock cash paid	$ 1.4	$ 1.4	$ 1.4