Retirement Plans - Weighted Average Assumptions Used in Calculating Benefit Obligation (Details)	Dec. 31, 2017	Dec. 31, 2016
United States
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Benefit Obligation [Abstract]
Discount rate	3.50%	4.00%
Rate of compensation increase	0.00%	0.00%
Germany
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Benefit Obligation [Abstract]
Discount rate	1.56%	1.60%
Rate of compensation increase	0.00%	0.00%