Business Acquisitions and Divestitures - Narrative (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended			12 Months Ended
	Feb. 28, 2017	Sep. 30, 2018	Jun. 30, 2018	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Feb. 23, 2017	Oct. 31, 2016
Business Acquisition [Line Items]
Reduction of earn-out liability		$ 1,600			$ 2,100
Fair value of contingent consideration liabilities at the date of acquisition					0	$ 5,954	$ 3,834
Payments for business acquisition					(19)	10,128	3,669
C&CA
Business Acquisition [Line Items]
Business combination, consideration transferred	$ 10,000
Contingent consideration, liability, maximum	8,000
Contingent consideration, liability	5,900				3,700
Fair value of contingent consideration liabilities at the date of acquisition	$ 900		$ (4,000)		4,000	900
Total cash paid								$ 9,968
C&CA | Recovery Audit Services – Americas | Audit Claim Recovery
Business Acquisition [Line Items]
Revenue					9,600	7,800
Income (loss) before income taxes due to acquisition					600	800
C&CA | Europe, Asia and Pacific | Audit Claim Recovery
Business Acquisition [Line Items]
Revenue					4,400	3,300
Income (loss) before income taxes due to acquisition					1,500	1,000
Lavante
Business Acquisition [Line Items]
Contingent consideration, liability, maximum									$ 4,500
Contingent consideration, liability					500				3,800
Fair value of contingent consideration liabilities at the date of acquisition		$ (1,600)		$ (2,100)	100	300
Revenue					2,200	1,900	400
Income (loss) before income taxes due to acquisition					$ (2,300)	$ (4,800)	(900)
Total cash paid									$ 3,809
Lavante | Other current liabilities
Business Acquisition [Line Items]
Contingent consideration, liability							2,000
Lavante | Other long-term liabilities
Business Acquisition [Line Items]
Contingent consideration, liability							$ 1,800