Share capital - Narrative (Details) - USD ($) $ / shares in Units, $ in Millions			12 Months Ended
	Dec. 27, 2021	Dec. 30, 2020	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	May 15, 2020
Class of Stock [Line Items]
Preferred stock, authorized			50,000,000
Preferred stock, par value			$ 0.001
Preferred stock, issued			0
Cash dividends paid	$ 12.6	$ 12.5
Dividends paid per share (in dollars per share)	$ 1.00	$ 1.00	$ 1.54	$ 1.48	$ 0.46
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, issued (in shares)			12,447,036	12,381,778
Common Stock, outstanding			12,447,036	12,381,778	12,698,703	12,636,523
Previous Program | Class A Stock
Class of Stock [Line Items]
Shares remaining available to be purchased (in shares)						98,625
New Program | Class A Stock
Class of Stock [Line Items]
Repurchase of class A common stock						530,000
Stock repurchased (as a percent)						4.20%
Shares remaining available to be purchased (in shares)			223,821	401,013		628,625
Stock repurchased and canceled (in shares)			177,192	718,522
Stock repurchased and canceled			$ 7.7	$ 15.0
Stock repurchased and canceled during period (in dollars per share)			$ 43.67	$ 20.94