Debt - Guarantees (Details) - 6 months ended Jun. 30, 2018 £ in Millions, $ in Millions	ZAR (R)	USD ($) guarantee	AUD ($) guarantee	ZAR (R) guarantee	GBP (£) guarantee
Geo Amey
Line of Credit Facility [Line Items]
Ownership percentage in South African Custodial Services Pty. Limited	50.00%
Working capital line of credit issued to GEOAmey		$ 15,800,000			£ 12.0
Note receivable for GEOAmey		$ 1,700,000			£ 1.3
Ravenhall
Line of Credit Facility [Line Items]
Number of letters of guarantee outstanding under separate international facilities relating to performance guarantees | guarantee		10	10	10	10
Letters of credit outstanding relating to performance guarantees		$ 15,100,000
Ravenhall | Letter of Credit | Revolver
Line of Credit Facility [Line Items]
Guaranteed obligations		74,100,000	$ 100.0
SACS
Line of Credit Facility [Line Items]
Guaranteed obligations		200,000		R 2,400,000
Remaining guarantee under letter of credit | R	R 2,400,000
Maximum loan amount under standby facility		1,500,000.0		R 20,000,000
Amount funded		$ 0