Debt - Narrative (Details) £ in Millions	6 Months Ended
	Jun. 30, 2017 GBP (£)	Jun. 30, 2017 USD ($)	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($)	Dec. 31, 2016 USD ($)
Entity's debt and other obligations [Line Items]
Amount outstanding				$ 645,400,000	$ 508,200,000
Proceeds from Lines of Credit		$ 861,800,000	$ 638,300,000
Repayments of Lines of Credit		733,800,000	634,400,000
Debt Instrument, Periodic Payment, Principal | £	£ 1.0
Accounts receivable securitization program
Entity's debt and other obligations [Line Items]
Maximum borrowing capacity				150,000,000
Amount outstanding				83,800,000	103,200,000
Proceeds from the accounts receivable securitization program		306,500,000	569,400,000
Payments in respect of accounts receivable securitization program		$ 325,900,000	$ 419,400,000
Revolving credit facilities
Entity's debt and other obligations [Line Items]
Maximum borrowing capacity				850,000,000
Amount outstanding				203,000,000	75,000,000
New Line of Credit Facilities
Entity's debt and other obligations [Line Items]
Maximum borrowing capacity				10,000,000
Amount outstanding				0	$ 0
U.K. Mortgage
Entity's debt and other obligations [Line Items]
Amount outstanding				$ 28,600,000
Debt Instrument, Face Amount | £	£ 23.0
Debt Instrument, Interest Rate, Stated Percentage	3.9025%			3.9025%