Actuarial Assumptions used to Compute Net Periodic Pension Cost (Details)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Compensation And Retirement Disclosure [Abstract]
Defined Benefit Plan, Sponsor Location [Extensible List]	us-gaap:ForeignPlanMember	us-gaap:ForeignPlanMember	us-gaap:ForeignPlanMember
Weighted-average discount rate	2.60%	3.80%	3.50%
Weighted-average long-term rate of return on plan assets	5.20%	5.30%	5.60%