Goodwill and Other Intangible Assets - Changes in Carrying Amount of Goodwill by Segment (Detail) - USD ($) $ in Thousands		9 Months Ended
	Jan. 04, 2018	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Goodwill [Line Items]
Goodwill, Amrop Acquisition		$ 5,102
Goodwill, Gross		182,209		$ 178,656
Goodwill, Impaired, Accumulated Impairment Loss		(59,764)		(59,764)
Goodwill [Roll Forward]
Foreign currency translation		(1,549)
Goodwill, Impairment Loss			$ (29,300)
Executive Search [Member]
Goodwill [Line Items]
Goodwill, Gross		145,952		142,399
Executive Search [Member] | Americas [Member]
Goodwill [Line Items]
Goodwill, Amrop Acquisition		0
Goodwill, Gross		88,595		88,690
Goodwill, Impaired, Accumulated Impairment Loss				0
Goodwill [Roll Forward]
Foreign currency translation		(95)
Executive Search [Member] | Europe [Member]
Goodwill [Line Items]
Goodwill, Amrop Acquisition		5,102
Goodwill, Gross		48,601		44,407
Goodwill, Impaired, Accumulated Impairment Loss				(23,507)
Goodwill [Roll Forward]
Foreign currency translation		(908)
Executive Search [Member] | Asia Pacific [Member]
Goodwill [Line Items]
Goodwill, Amrop Acquisition		0
Goodwill, Gross		8,756		9,302
Goodwill, Impaired, Accumulated Impairment Loss				0
Goodwill [Roll Forward]
Foreign currency translation		(546)
Heidrick Consulting [Domain]
Goodwill [Line Items]
Goodwill, Amrop Acquisition		0
Goodwill, Gross		36,257		36,257
Goodwill, Impaired, Accumulated Impairment Loss				$ (36,257)
Goodwill [Roll Forward]
Foreign currency translation		$ 0
Amrop [Member] [Domain]
Goodwill [Line Items]
Goodwill, Acquired During Period	$ 5,100