Special Charges - Schedule of Special Charges By Segment (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Restructuring Cost and Reserve [Line Items]
Special charges	$ 0	$ 40,885	$ 10,445
Operating Segments
Restructuring Cost and Reserve [Line Items]
Special charges		37,207	9,833
Operating Segments | Corporate Finance & Restructuring
Restructuring Cost and Reserve [Line Items]
Special charges		5,440	0
Operating Segments | Forensic and Litigation Consulting
Restructuring Cost and Reserve [Line Items]
Special charges		12,334	2,304
Operating Segments | Economic Consulting
Restructuring Cost and Reserve [Line Items]
Special charges		6,624	0
Operating Segments | Technology
Restructuring Cost and Reserve [Line Items]
Special charges		5,057	7,529
Operating Segments | Strategic Communications
Restructuring Cost and Reserve [Line Items]
Special charges		7,752	0
Unallocated Corporate
Restructuring Cost and Reserve [Line Items]
Special charges		$ 3,678	$ 612