Defined Benefit Plans - Narrative (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) plan	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Domestic
Defined Benefit Plan Disclosure [Line Items]
Estimated unrecognized loss that will be amortized from AOCI, net of income taxes	$ 200
Number of plans | plan	2
Benefit obligation	$ 7,200	$ 7,000
Fair value of plan assets	$ 5,500	$ 5,000
Annual rate of return on plan assets	7.00%	7.00%
Estimated future benefit payments	$ 400
Maximum | Domestic
Defined Benefit Plan Disclosure [Line Items]
Discount rate	3.50%	4.00%
Minimum | Domestic
Defined Benefit Plan Disclosure [Line Items]
Discount rate	3.50%	3.80%
United Kingdom Subsidiary | UK Plan
Defined Benefit Plan Disclosure [Line Items]
Underfunded status of UK Plan	$ 36,650	$ 49,495
Annual rate of inflation	2.10%	2.20%
Average life expectancy	26 years
Estimated unrecognized loss that will be amortized from AOCI, net of income taxes	$ 2,100
Future estimated employer contributions to be paid next year	4,800
Benefit obligation	332,618	$ 306,731	$ 295,825
Fair value of plan assets	$ 295,968	$ 257,236	$ 263,555
Discount rate	2.50%	2.70%
Annual rate of return on plan assets	5.30%	6.20%	6.30%
Selling, General and Administrative Expenses | Domestic
Defined Benefit Plan Disclosure [Line Items]
Reclassification adjustment from AOCI, net of income taxes	$ 200	$ 200	$ 200
Selling, General and Administrative Expenses | United Kingdom Subsidiary | UK Plan
Defined Benefit Plan Disclosure [Line Items]
Reclassification adjustment from AOCI, net of income taxes	2,300	1,700	$ 2,000
Other Noncurrent Liabilities | United Kingdom Subsidiary | UK Plan
Defined Benefit Plan Disclosure [Line Items]
Underfunded status of UK Plan	$ 36,700	$ 49,500