Goodwill and Other Intangible Assets - Summary of Changes in Carrying Amounts of Goodwill by Operating Segment (Detail) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Goodwill [Roll Forward]
Goodwill	$ 1,398,942	$ 1,374,140	$ 1,392,437
Accumulated goodwill impairment	(194,139)	(194,139)	(194,139)
Goodwill beginning of period	1,180,001	1,198,298
Acquisitions	11,900	218
Foreign currency translation adjustment and other	12,902	(18,515)
Goodwill end of period	1,204,803	1,180,001
Corporate Finance & Restructuring
Goodwill [Roll Forward]
Goodwill	454,816	440,666	441,548
Goodwill beginning of period	440,666	441,548
Acquisitions	11,900
Foreign currency translation adjustment and other	2,250	(882)
Goodwill end of period	454,816	440,666
Forensic and Litigation Consulting
Goodwill [Roll Forward]
Goodwill	233,719	230,544	235,211
Goodwill beginning of period	230,544	235,211
Foreign currency translation adjustment and other	3,175	(4,667)
Goodwill end of period	233,719	230,544
Economic Consulting
Goodwill [Roll Forward]
Goodwill	268,995	268,209	269,341
Goodwill beginning of period	268,209	269,341
Foreign currency translation adjustment and other	786	(1,132)
Goodwill end of period	268,995	268,209
Technology
Goodwill [Roll Forward]
Goodwill	117,740	117,607	117,888
Goodwill beginning of period	117,607	117,888
Foreign currency translation adjustment and other	133	(281)
Goodwill end of period	117,740	117,607
Strategic Communications
Goodwill [Roll Forward]
Goodwill	323,672	317,114	328,449
Accumulated goodwill impairment	(194,139)	(194,139)	$ (194,139)
Goodwill beginning of period	122,975	134,310
Acquisitions	0	218
Foreign currency translation adjustment and other	6,558	(11,553)
Goodwill end of period	129,533	$ 122,975
Restructuring Business
Goodwill [Roll Forward]
Initial payment	8,900
Restructuring Business \| Corporate Finance & Restructuring
Goodwill [Roll Forward]
Acquisitions	$ 11,900

Business Combination, Consideration Transferred, Initial Payment

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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.

+ References

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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Amount before 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.

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

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