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, 2018	Dec. 31, 2017	Dec. 31, 2016
Goodwill [Roll Forward]
Goodwill	$ 1,398,942	$ 1,374,140
Accumulated goodwill impairment	(194,139)	(194,139)
Goodwill beginning of period	1,204,803	1,180,001
Sale of business	(20,928)
Acquisitions		11,900
Foreign currency translation adjustment and other	(11,559)	12,902
Goodwill	1,366,455	1,398,942	$ 1,374,140
Accumulated goodwill impairment	(194,139)	(194,139)	(194,139)
Goodwill end of period	1,172,316	1,204,803	1,180,001
Proceeds from sale of business	50,283	0	0
Corporate Finance
Goodwill [Roll Forward]
Goodwill	454,816	440,666
Goodwill beginning of period	454,816	440,666
Acquisitions		11,900
Foreign currency translation adjustment and other	(3,819)	2,250
Goodwill	450,997	454,816	440,666
Goodwill end of period	450,997	454,816	440,666
Forensic and Litigation Consulting
Goodwill [Roll Forward]
Goodwill	233,719	230,544
Goodwill beginning of period	233,719	230,544
Foreign currency translation adjustment and other	(2,182)	3,175
Goodwill	231,537	233,719	230,544
Goodwill end of period	231,537	233,719	230,544
Economic Consulting
Goodwill [Roll Forward]
Goodwill	268,995	268,209
Goodwill beginning of period	268,995	268,209
Foreign currency translation adjustment and other	(448)	786
Goodwill	268,547	268,995	268,209
Goodwill end of period	268,547	268,995	268,209
Technology
Goodwill [Roll Forward]
Goodwill	117,740	117,607
Goodwill beginning of period	117,740	117,607
Sale of business	(20,928)
Acquisitions	20,900
Foreign currency translation adjustment and other	(89)	133
Goodwill	96,723	117,740	117,607
Goodwill end of period	96,723	117,740	117,607
Proceeds from sale of business	50,300	8,900
Strategic Communications
Goodwill [Roll Forward]
Goodwill	323,672	317,114
Accumulated goodwill impairment	(194,139)	(194,139)
Goodwill beginning of period	129,533	122,975
Foreign currency translation adjustment and other	(5,021)	6,558
Goodwill	318,651	323,672	317,114
Accumulated goodwill impairment	(194,139)	(194,139)	(194,139)
Goodwill end of period	$ 124,512	129,533	$ 122,975
Restructuring Business \| Technology
Goodwill [Roll Forward]
Sale of business		$ (11,900)

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.

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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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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 divestiture 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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The cash inflow associated with the amount received from the sale of a portion of the company's business, for example a segment, division, branch or other business, during the period.

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