Summary of significant accounting policies, Part 1 (Details)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Buildings [Member] | Minimum [Member]
Property, Plant and Equipment [Abstract]
Estimated useful lives	31 years
Buildings [Member] | Maximum [Member]
Property, Plant and Equipment [Abstract]
Estimated useful lives	39 years
Building Improvements [Member] | Minimum [Member]
Property, Plant and Equipment [Abstract]
Estimated useful lives	10 years
Building Improvements [Member] | Maximum [Member]
Property, Plant and Equipment [Abstract]
Estimated useful lives	39 years
Furniture and Equipment [Member] | Minimum [Member]
Property, Plant and Equipment [Abstract]
Estimated useful lives	3 years
Furniture and Equipment [Member] | Maximum [Member]
Property, Plant and Equipment [Abstract]
Estimated useful lives	15 years
Software [Member]
Property, Plant and Equipment [Abstract]
Estimated useful lives	3-7 years or product life
Leasehold Improvements [Member]
Property, Plant and Equipment [Abstract]
Estimated useful lives	Lesser of the asset life or lease term
Revenue [Member] | U.S. Government [Member]
Concentration Risk [Abstract]
Concentration risk percentage	76.00%	78.00%	86.00%
Accounts Receivable [Member] | U.S. Government [Member]
Concentration Risk [Abstract]
Concentration risk percentage	76.00%	89.00%