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Earnings per share (Details) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 29, 2020 USD ($) $ / shares shares	Mar. 31, 2019 USD ($) $ / shares shares	Mar. 29, 2020 £ / shares	Dec. 31, 2019 £ / shares	Mar. 31, 2019 £ / shares
Basic earnings:
Net income / (loss) \| $	$ 6.2	$ (3.8)
Weighted average number of £0.50 ordinary shares:
For basic earnings per share (shares)	27,440,423	27,032,677
Dilutive effect of potential common stock (shares)	453,635	0
For diluted earnings per share (shares)	27,894,058	27,032,677
Earnings / (loss) per share using weighted average number of ordinary shares outstanding:
Basic earnings per ordinary share (usd per share) \| $ / shares	$ 0.23	$ (0.14)
Diluted earnings per ordinary share (usd per share) \| $ / shares	$ 0.22	$ (0.14)
Class of Stock [Line Items]
Shares not included in the computation of diluted EPS (in shares)	735,730
Ordinary shares
Class of Stock [Line Items]
Common stock, par value (in GBP per share) \| £ / shares			£ 0.50	£ 0.50	£ 0.50