Summary of significant accounting policies (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Accounting Policies [Abstract]
Short-term investments	$ 0	$ 0
Allowance for doubtful accounts	2,100	$ 2,000
Cumulative effect adjustment, net of tax	$ 6,500
Furniture and Equipment [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Estimated useful life	7 years
Furniture and Equipment [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Estimated useful life	10 years
Electronic Data Processing Hardware and Software [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Estimated useful life	3 years
Electronic Data Processing Hardware and Software [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Estimated useful life	7 years
Leasehold Improvements [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Estimated useful life	2 years
Leasehold Improvements [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Estimated useful life	6 years