Accounting policies (Details)	12 Months Ended
	Dec. 31, 2014
Buildings [Member] | Maximum [Member]
Property, Plant and Equipment, Useful Life	40 years
Buildings [Member] | Minimum [Member]
Property, Plant and Equipment, Useful Life	5 years
Plant and equipment [Member] | Maximum [Member]
Property, Plant and Equipment, Useful Life	40 years
Plant and equipment [Member] | Minimum [Member]
Property, Plant and Equipment, Useful Life	4 years
Distribution system [Member] | Maximum [Member]
Property, Plant and Equipment, Useful Life	40 years
Distribution system [Member] | Minimum [Member]
Property, Plant and Equipment, Useful Life	3 years
Furniture and Fixtures [Member] | Maximum [Member]
Property, Plant and Equipment, Useful Life	10 years
Furniture and Fixtures [Member] | Minimum [Member]
Property, Plant and Equipment, Useful Life	3 years
Vehicles [Member] | Maximum [Member]
Property, Plant and Equipment, Useful Life	10 years
Vehicles [Member] | Minimum [Member]
Property, Plant and Equipment, Useful Life	3 years
Leasehold improvements [Member]
Property, Plant and Equipment, Estimated Useful Lives	Shorter of 5 years or lease term
Lab equipment [Member]. | Maximum [Member]
Property, Plant and Equipment, Useful Life	10 years
Lab equipment [Member]. | Minimum [Member]
Property, Plant and Equipment, Useful Life	5 years