Summary of Changes in Carrying Amounts of Goodwill by Reportable Segment (Detail) - USD ($) $ in Thousands	9 Months Ended
Sep. 30, 2017	Dec. 31, 2016
Goodwill [Line Items]
Goodwill, Gross	$ 1,398,303	$ 1,374,140
Accumulated goodwill impairment	(194,139)	(194,139)
Goodwill, net	1,204,164	1,180,001
Acquisitions	11,900
Foreign currency translation adjustment and other	12,263
Corporate Finance & Restructuring
Goodwill [Line Items]
Goodwill, Gross	454,858	440,666
Goodwill, net	454,858	440,666
Acquisitions	11,900
Foreign currency translation adjustment and other	2,292
Forensic and Litigation Consulting
Goodwill [Line Items]
Goodwill, Gross	233,511	230,544
Goodwill, net	233,511	230,544
Foreign currency translation adjustment and other	2,967
Economic Consulting
Goodwill [Line Items]
Goodwill, Gross	268,921	268,209
Goodwill, net	268,921	268,209
Foreign currency translation adjustment and other	712
Technology
Goodwill [Line Items]
Goodwill, Gross	117,729	117,607
Goodwill, net	117,729	117,607
Foreign currency translation adjustment and other	122
Strategic Communications
Goodwill [Line Items]
Goodwill, Gross	323,284	317,114
Accumulated goodwill impairment	(194,139)	(194,139)
Goodwill, net	129,145	$ 122,975
Foreign currency translation adjustment and other	$ 6,170

Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13816-109267

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6388280&loc=d3e13770-109266

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 45
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=77989000&loc=SL49117168-202975

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