Goodwill and Other Intangible Assets - Summary of Changes in Carrying Amounts of Goodwill by Operating Segment (Detail) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
Dec. 31, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Goodwill [Roll Forward]
Goodwill		$ 1,366,455	$ 1,398,942
Accumulated goodwill impairment		(194,139)	(194,139)
Goodwill beginning of period		1,172,316	1,204,803
Sale of business			(20,928)
Acquisitions		27,389
Foreign currency translation adjustment and other		3,062	(11,559)
Goodwill	$ 1,396,906	1,396,906	1,366,455	$ 1,398,942
Accumulated goodwill impairment	(194,139)	(194,139)	(194,139)	(194,139)
Goodwill end of period	1,202,767	1,202,767	1,172,316	1,204,803
Proceeds from sale of business		0	50,283	0
Corporate Finance
Goodwill [Roll Forward]
Goodwill		450,997	454,816
Accumulated goodwill impairment		0	0
Goodwill beginning of period		450,997	454,816
Sale of business			0
Acquisitions		27,389
Foreign currency translation adjustment and other		456	(3,819)
Goodwill	478,842	478,842	450,997	454,816
Accumulated goodwill impairment	0	0	0	0
Goodwill end of period	478,842	478,842	450,997	454,816
Forensic and Litigation Consulting
Goodwill [Roll Forward]
Goodwill		231,537	233,719
Accumulated goodwill impairment		0	0
Goodwill beginning of period		231,537	233,719
Sale of business			0
Acquisitions		0
Foreign currency translation adjustment and other		583	(2,182)
Goodwill	232,120	232,120	231,537	233,719
Accumulated goodwill impairment	0	0	0	0
Goodwill end of period	232,120	232,120	231,537	233,719
Economic Consulting
Goodwill [Roll Forward]
Goodwill		268,547	268,995
Accumulated goodwill impairment		0	0
Goodwill beginning of period		268,547	268,995
Sale of business			0
Acquisitions		0
Foreign currency translation adjustment and other		130	(448)
Goodwill	268,677	268,677	268,547	268,995
Accumulated goodwill impairment	0	0	0	0
Goodwill end of period	268,677	268,677	268,547	268,995
Technology
Goodwill [Roll Forward]
Goodwill		96,723	117,740
Accumulated goodwill impairment		0	0
Goodwill beginning of period		96,723	117,740
Sale of business			(20,928)
Acquisitions		0
Foreign currency translation adjustment and other		47	(89)
Goodwill	96,770	96,770	96,723	117,740
Accumulated goodwill impairment	0	0	0	0
Goodwill end of period	96,770	96,770	96,723	117,740
Proceeds from sale of business			50,300
Strategic Communications
Goodwill [Roll Forward]
Goodwill		318,651	323,672
Accumulated goodwill impairment		(194,139)	(194,139)
Goodwill beginning of period		124,512	129,533
Sale of business			0
Acquisitions		0
Foreign currency translation adjustment and other		1,846	(5,021)
Goodwill	320,497	320,497	318,651	323,672
Accumulated goodwill impairment	(194,139)	(194,139)	(194,139)	(194,139)
Goodwill end of period	126,358	$ 126,358	$ 124,512	$ 129,533
Andersch AG
Goodwill [Roll Forward]
Acquisitions	$ 27,400

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 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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