Restructuring (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2017	Dec. 31, 2018
Restructuring Cost and Reserve [Line Items]
Restructuring Reserve	$ 3,775		$ 1,286
Restructuring Charges	4,130	$ 15,700
Payments for Restructuring	(1,554)
Restructuring Charges, Non-cash Write-offs	17
Other Restructuring Costs	(4)
Restructuring Reserve, Foreign Currency Translation Gain (Loss)	(74)
Employee Severance [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring Reserve	3,775		1,269
Restructuring Charges	4,130
Payments for Restructuring	(1,554)
Restructuring Charges, Non-cash Write-offs	0
Other Restructuring Costs	(4)
Restructuring Reserve, Foreign Currency Translation Gain (Loss)	(74)
Other Professional and Consulting Fees [Domain]
Restructuring Cost and Reserve [Line Items]
Restructuring Reserve	0		$ 17
Restructuring Charges	0
Payments for Restructuring	0
Restructuring Charges, Non-cash Write-offs	17
Other Restructuring Costs	0
Restructuring Reserve, Foreign Currency Translation Gain (Loss)	0
Corporate, Non-Segment [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring Charges	100
Americas Segment [Member] | Operating Segments [Member] | Executive Search [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring Charges	$ 4,100