Commitments and Contingencies - Additional Information (Details) £ in Millions, $ in Millions		9 Months Ended
	Sep. 30, 2018 USD ($) $ / £	Sep. 30, 2018 USD ($) $ / £	Sep. 30, 2018 GBP (£) $ / £
Commitments And Contingencies [Line Items]
Purchase Commitments	$ 11.9	$ 11.9
Long-term purchase commitment, Amount	130.0
Pension Plan, Defined Benefit
Commitments And Contingencies [Line Items]
Defined benefit plan, estimated future employer contributions in current fiscal year, description		We have a contributory defined benefit plan that covers certain employees in the United Kingdom. As of September 30, 2018, the unfunded liability for this defined benefit plan was approximately $25.1 million. We are obligated to make annual contributions, each year through December 2029, of approximately GBP 2 million (approximately $2.6 million based on a GBP: USD exchange rate of 1.3:1). The trustees are required to review the funding position every three years, and the most recent review was carried out as of April 5, 2016. The outcome of a review can result in a change in the amount of the payment.
Defined benefit plan, expected future benefit payments in year one	2.6	$ 2.6	£ 2
Defined benefit plan, expected future benefit payments in year two	2.6	2.6	2
Defined benefit plan, expected future benefit payments in year three	2.6	2.6	2
Defined benefit plan, expected future benefit payments in year four	2.6	2.6	2
Defined benefit plan, expected future benefit payments in year five	2.6	2.6	2
Defined benefit plan, expected future benefit payments in eight fiscal years thereafter	$ 2.6	$ 2.6	£ 2
GBP:USD exchange rate | $ / £	1.3	1.3	1.3
Unfunded liability of defined benefit plan	$ 25.1	$ 25.1