Summary of significant accounting policies (Details 2)	12 Months Ended
	Dec. 31, 2012
Buildings
Property, Plant and Equipment [Line Items]
Estimated useful life, minimum (in years)	25
Estimated useful life, maximum (in years)	33
Computer equipment
Property, Plant and Equipment [Line Items]
Estimated useful life, minimum (in years)	2
Estimated useful life, maximum (in years)	5
Office furniture and other equipment.
Property, Plant and Equipment [Line Items]
Estimated useful life, minimum (in years)	2
Estimated useful life, maximum (in years)	5