Debt - Guarantees (Details) - 3 months ended Mar. 31, 2018 £ in Millions, $ in Millions	ZAR (R)	AUD ($) guarantee	ZAR (R) guarantee	USD ($) 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				$ 16,900,000	£ 12.0
Note receivable for GEOAmey				$ 1,800,000	£ 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				$ 15,600,000
Ravenhall | Letter of Credit | Revolver
Line of Credit Facility [Line Items]
Guaranteed obligations		$ 100.0		77,000,000
SACS
Line of Credit Facility [Line Items]
Guaranteed obligations			R 2,400,000	200,000
Remaining guarantee under letter of credit | R	R 2,400,000
Maximum loan amount under standby facility			R 20,000,000	1,700,000.0
Amount funded				$ 0