Share capital - Narrative (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2022	Feb. 28, 2022	Dec. 31, 2021	Mar. 31, 2021	Dec. 31, 2020	May 15, 2020
Class of Stock [Line Items]
Preferred stock, authorized (in shares)	50,000,000
Preferred stock, par value (in dollars per share)	$ 0.001
Preferred stock, issued (in shares)	0
Class A Stock
Class of Stock [Line Items]
Common stock, authorized (in shares)	50,000,000		50,000,000
Common stock, par value (in dollars per share)	$ 0.001		$ 0.001
Common stock, outstanding (in shares)	12,156,174	12,322,073	12,447,036	12,586,043	12,381,778	12,636,523
Common stock, issued (in shares)	12,156,174		12,447,036
Class B Stock
Class of Stock [Line Items]
Common stock, authorized (in shares)	99,665		99,665
Common stock, par value (in dollars per share)	$ 0.001		$ 0.001
Common stock, outstanding (in shares)	99,665		99,665
Common stock, issued (in shares)	99,665		99,665
Previous Program | Class A Stock
Class of Stock [Line Items]
Remaining amount authorized for repurchase (in shares)		12,407				98,625
New Program | Class A Stock
Class of Stock [Line Items]
Repurchase of class A common stock (in shares)		518,000				530,000
Shares repurchased as a percent of shares outstanding (as a percent)		4.20%				4.20%
Remaining amount authorized for repurchase (in shares)	364,508	530,407				628,625
Stock repurchased (in shares)	377,313
Total consideration for repurchase of stock	$ 16.2
Stock repurchased (in dollars per share)	$ 42.82