Summary of significant accounting policies (Details 2)	12 Months Ended
	Dec. 31, 2014
Buildings | Maximum
Property, Plant and Equipment [Line Items]
Estimated useful life	33 years
Buildings | Minimum
Property, Plant and Equipment [Line Items]
Estimated useful life	25 years
Computer equipment | Maximum
Property, Plant and Equipment [Line Items]
Estimated useful life	5 years
Computer equipment | Minimum
Property, Plant and Equipment [Line Items]
Estimated useful life	2 years
Office furniture and other equipment. | Maximum
Property, Plant and Equipment [Line Items]
Estimated useful life	5 years
Office furniture and other equipment. | Minimum
Property, Plant and Equipment [Line Items]
Estimated useful life	2 years