Earnings per share - Summary of Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Earnings Per Share [Abstract]
Basic weighted average number of shares outstanding	12,642,306	12,642,576	12,904,397
Net dilutive effect of share-based awards, treasury method (in shares)	940,522	574,759	947,435
Diluted weighted average number of shares outstanding	13,582,828	13,217,335	13,851,832
Net income	$ 158,964	$ 122,986	$ 52,953
Basic (in dollars per share)	$ 12.57	$ 9.73	$ 4.10
Diluted (in dollars per share)	$ 11.70	$ 9.30	$ 3.82