Fair value measurements - level 3 roll (Details) - Measured at fair value on a recurring basis: - Contingent Consideration Classified as Equity [Member] - Fair Value, Inputs, Level 3 [Member] - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Oct. 01, 2016	Sep. 26, 2015	Oct. 01, 2016	Sep. 26, 2015
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	$ 44.9	$ 0.0	$ 17.9	$ 12.4
Net realized (gains) losses	(0.4)	0.0	(4.0)	(12.4)
Purchases or additions	30.6	0.0	61.1	0.0
Foreign currency effect	0.0	0.0	(0.1)	0.0
Settlements	0.1	0.0	0.1	0.0
Ending balance	$ 75.0	$ 0.0	$ 75.0	$ 0.0