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Summary of Significant Accounting Policies - Risks and Uncertainties and Use of Estimates (Details) | 12 Months Ended
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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 |  |

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