Goodwill and Other Intangible Assets - Summary of Changes in Carrying Amounts of Goodwill by Operating Segment (Detail) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2018	Sep. 30, 2018	Dec. 31, 2017
Goodwill [Roll Forward]
Goodwill	$ 1,370,068	$ 1,370,068	$ 1,398,942
Accumulated goodwill impairment	(194,139)	(194,139)	(194,139)
Goodwill beginning of period		1,204,803
Sale of business (1)		(20,928)
Foreign currency translation adjustment and other		(7,946)
Goodwill end of period	1,175,929	1,175,929
Corporate Finance & Restructuring
Goodwill [Roll Forward]
Goodwill	451,756	451,756	454,816
Goodwill beginning of period		454,816
Sale of business (1)		0
Foreign currency translation adjustment and other		(3,060)
Goodwill end of period	451,756	451,756
Forensic and Litigation Consulting
Goodwill [Roll Forward]
Goodwill	232,204	232,204	233,719
Goodwill beginning of period		233,719
Sale of business (1)		0
Foreign currency translation adjustment and other		(1,515)
Goodwill end of period	232,204	232,204
Economic Consulting
Goodwill [Roll Forward]
Goodwill	268,719	268,719	268,995
Goodwill beginning of period		268,995
Sale of business (1)		0
Foreign currency translation adjustment and other		(276)
Goodwill end of period	268,719	268,719
Technology
Goodwill [Roll Forward]
Goodwill	96,760	96,760	117,740
Goodwill beginning of period		117,740
Sale of business (1)		(20,928)
Foreign currency translation adjustment and other		(52)
Goodwill end of period	96,760	96,760
Strategic Communications
Goodwill [Roll Forward]
Goodwill	320,629	320,629	323,672
Accumulated goodwill impairment	(194,139)	(194,139)	$ (194,139)
Goodwill beginning of period		129,533
Sale of business (1)		0
Foreign currency translation adjustment and other		(3,043)
Goodwill end of period	126,490	$ 126,490
Restructuring Business \| Technology
Goodwill [Roll Forward]
Sale of business (1)	$ (20,900)

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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Amount of foreign currency translation gain (loss) which increases (decreases) an asset representing future economic benefits from other assets acquired in a business combination that are not individually identified and separately recognized.

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