Commitments and contingencies (Details) $ in Millions	Jun. 28, 2017 USD ($)	May 31, 2017 lawsuit
Loss Contingencies [Line Items]
Number of lawsuits filed		2
Court of Chancery of the State of Delaware
Loss Contingencies [Line Items]
Number of lawsuits filed		8
State of Pennsylvania in the Court of Common Pleas of Cumberland County
Loss Contingencies [Line Items]
Number of lawsuits filed		1
Middle District of Pennsylvania
Loss Contingencies [Line Items]
Number of lawsuits filed		1
Subsequent Event
Loss Contingencies [Line Items]
Early contract termination fees | $	$ 325
Maximum | Subsequent Event
Loss Contingencies [Line Items]
Maximum reimbursement of transaction costs | $	$ 25