Goodwill and Other Intangible Assets - Summary of Changes in Carrying Amounts of Goodwill by Operating Segment (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Goodwill [Roll Forward]
Goodwill	$ 1,402,150	$ 1,398,942
Accumulated goodwill impairment	(194,139)	(194,139)
Goodwill beginning of period	1,204,803
Foreign currency translation adjustment and other	3,208
Goodwill end of period	1,208,011
Corporate Finance & Restructuring
Goodwill [Roll Forward]
Goodwill	453,914	454,816
Goodwill beginning of period	454,816
Foreign currency translation adjustment and other	(902)
Goodwill end of period	453,914
Forensic and Litigation Consulting
Goodwill [Roll Forward]
Goodwill	234,963	233,719
Goodwill beginning of period	233,719
Foreign currency translation adjustment and other	1,244
Goodwill end of period	234,963
Economic Consulting
Goodwill [Roll Forward]
Goodwill	269,289	268,995
Goodwill beginning of period	268,995
Foreign currency translation adjustment and other	294
Goodwill end of period	269,289
Technology
Goodwill [Roll Forward]
Goodwill	117,800	117,740
Goodwill beginning of period	117,740
Foreign currency translation adjustment and other	60
Goodwill end of period	117,800
Strategic Communications
Goodwill [Roll Forward]
Goodwill	326,184	323,672
Accumulated goodwill impairment	(194,139)	$ (194,139)
Goodwill beginning of period	129,533
Foreign currency translation adjustment and other	2,512
Goodwill end of period	$ 132,045