Summary of significant accounting policies - Useful Life of PPE and Intangible Assets (Details)	12 Months Ended
	Dec. 31, 2017
Minimum
Property, Plant and Equipment
Finite lived intangible assets	5 years
Maximum
Property, Plant and Equipment
Finite lived intangible assets	20 years
Buildings and leasehold equipment | Minimum
Property, Plant and Equipment
PPE useful life (in years)	20 years
Buildings and leasehold equipment | Maximum
Property, Plant and Equipment
PPE useful life (in years)	40 years
Machinery and equipment | Minimum
Property, Plant and Equipment
PPE useful life (in years)	5 years
Machinery and equipment | Maximum
Property, Plant and Equipment
PPE useful life (in years)	30 years
Capitalized software | Minimum
Property, Plant and Equipment
PPE useful life (in years)	1 year
Capitalized software | Maximum
Property, Plant and Equipment
PPE useful life (in years)	7 years