Special Charges - Schedule of Special Charges By Segment (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Restructuring Cost and Reserve [Line Items]
Special charges	$ 40,885,000	$ 10,445,000	$ 0
Operating Segments
Restructuring Cost and Reserve [Line Items]
Special charges	37,207,000	9,833,000
Operating Segments | Corporate Finance & Restructuring
Restructuring Cost and Reserve [Line Items]
Special charges	5,440,000	0
Operating Segments | Forensic and Litigation Consulting
Restructuring Cost and Reserve [Line Items]
Special charges	12,334,000	2,304,000
Operating Segments | Economic Consulting
Restructuring Cost and Reserve [Line Items]
Special charges	6,624,000	0
Operating Segments | Technology
Restructuring Cost and Reserve [Line Items]
Special charges	5,057,000	7,529,000
Operating Segments | Strategic Communications
Restructuring Cost and Reserve [Line Items]
Special charges	7,752,000	0
Unallocated Corporate
Restructuring Cost and Reserve [Line Items]
Special charges	$ 3,678,000	$ 612,000