Goodwill and Other Intangible Assets - Summary of Changes in Carrying Amounts of Goodwill by Operating Segment (Detail) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Goodwill [Roll Forward]
Goodwill	$ 1,392,871	$ 1,398,942
Accumulated goodwill impairment	(194,139)	(194,139)
Goodwill beginning of period	1,204,803
Foreign currency translation adjustment and other	(6,071)
Goodwill end of period	1,198,732
Corporate Finance & Restructuring
Goodwill [Roll Forward]
Goodwill	452,362	454,816
Goodwill beginning of period	454,816
Foreign currency translation adjustment and other	(2,454)
Goodwill end of period	452,362
Forensic and Litigation Consulting
Goodwill [Roll Forward]
Goodwill	232,551	233,719
Goodwill beginning of period	233,719
Foreign currency translation adjustment and other	(1,168)
Goodwill end of period	232,551
Economic Consulting
Goodwill [Roll Forward]
Goodwill	268,782	268,995
Goodwill beginning of period	268,995
Foreign currency translation adjustment and other	(213)
Goodwill end of period	268,782
Technology
Goodwill [Roll Forward]
Goodwill	117,702	117,740
Goodwill beginning of period	117,740
Foreign currency translation adjustment and other	(38)
Goodwill end of period	117,702
Strategic Communications
Goodwill [Roll Forward]
Goodwill	321,474	323,672
Accumulated goodwill impairment	(194,139)	$ (194,139)
Goodwill beginning of period	129,533
Foreign currency translation adjustment and other	(2,198)
Goodwill end of period	$ 127,335