Debt - Guarantees (Details) - 9 months ended Sep. 30, 2017 £ in Millions, CAD in Millions, AUD in Millions	ZAR	AUD guarantee	USD ($) guarantee	ZAR guarantee	CAD 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			$ 16,100,000			£ 12.0
Note receivable for GEOAmey			$ 3,400,000			£ 2.5
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	10
Letters of credit outstanding relating to performance guarantees			$ 15,900,000
Ravenhall | Letter of Credit | Revolver
Line of Credit Facility [Line Items]
Guaranteed obligations		AUD 100.0	78,300,000
SACS
Line of Credit Facility [Line Items]
Guaranteed obligations			200,000	ZAR 2,400,000
Remaining guarantee under letter of credit | ZAR	ZAR 2,400,000
Maximum loan amount under standby facility			1,500,000.0	ZAR 20,000,000
Amount funded			$ 0
Canada Facility
Line of Credit Facility [Line Items]
Potential estimated exposure of tax obligations | CAD					CAD 1.4