Derivative Financial Instruments - Narrative (Details) - 12 months ended Dec. 31, 2017 $ in Millions	USD ($)	GBP (£)
Facility and 2012 Term Loan | Net Investment Hedging
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Notional amount | £		£ 150,000,000
Cross Currency Interest Rate Contract | Net Investment Hedging
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Notional amount | £		£ 105,000,000
Maximum
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Estimate change in fair value of derivative for assumption of one percentage point change in the interest rate | $	$ 2
Maximum | Cross Currency Interest Rate Contract
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Estimate change in fair value of derivative for assumption of one percentage point change in the interest rate | $	$ 2