Accounting policies (Details)	12 Months Ended
	Dec. 31, 2012
Building [Member] | Minimum [Member]
Property, Plant and Equipment, Useful Life	5 years
Building [Member] | Maximum [Member]
Property, Plant and Equipment, Useful Life	40 years
Property, Plant and Equipment, Other Types [Member] | Minimum [Member]
Property, Plant and Equipment, Useful Life	4 years
Property, Plant and Equipment, Other Types [Member] | Maximum [Member]
Property, Plant and Equipment, Useful Life	40 years
Distribution System [Member] | Minimum [Member]
Property, Plant and Equipment, Useful Life	3 years
Distribution System [Member] | Maximum [Member]
Property, Plant and Equipment, Useful Life	40 years
Office Furniture Fixtures and Equipment [Member] | Minimum [Member]
Property, Plant and Equipment, Useful Life	3 years
Office Furniture Fixtures and Equipment [Member] | Maximum [Member]
Property, Plant and Equipment, Useful Life	10 years
Vehicles [Member] | Minimum [Member]
Property, Plant and Equipment, Useful Life	3 years
Vehicles [Member] | Maximum [Member]
Property, Plant and Equipment, Useful Life	10 years
Leasehold Improvements [Member]
Property, Plant and Equipment, Estimated Useful Lives	Shorter of 5 years or operating lease term outstanding
Lab Equipment [Member] | Minimum [Member]
Property, Plant and Equipment, Useful Life	5 years
Lab Equipment [Member] | Maximum [Member]
Property, Plant and Equipment, Useful Life	10 years