Summary of Significant Accounting Policies - Risks and Uncertainties and Use of Estimates (Details)	12 Months Ended
	Dec. 31, 2017 employee customer	Dec. 31, 2016 customer	Dec. 31, 2015 customer
Segment Reporting Information [Line Items]
Number of employees	8,600
Percent of employees located outside of the U.S.	15.00%
Percent of employees covered by collective bargaining agreements	40.00%
Workforce Subject to Collective Bargaining Arrangements That Have Expired [Member]
Segment Reporting Information [Line Items]
Number of employees	600
Sales Revenue [Member]
Segment Reporting Information [Line Items]
Number of customers \| customer	0	0	0
Workforce Subject to Collective Bargaining Arrangements Expiring within One Year [Member]
Segment Reporting Information [Line Items]
Number of employees	300