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Commitments and Contingencies - Legal Contingencies (Details) - USD ($)
 1 Months Ended 12 Months Ended
Aug. 18, 2017
Sep. 30, 2017
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2013
Loss Contingencies [Line Items]            
Gain recognized     $ 0 $ 5,500,000 [1] $ (3,800,000) [1]  
Ravenswood Retiree Medical Benefits Changes [Member]            
Loss Contingencies [Line Items]            
Litigation settlement amount $ 23,000,000          
Litigation settlement period     10 years      
Ravenswood retiree legal settlement   $ 5,000,000        
Gain recognized   5,500,000        
Loss contingency accrual   $ 12,500,000        
Other liabilities, current     $ 2,000,000      
Other liabilities     9,800,000      
Litigation settlement, annual installment     $ 2,000,000      
Litigation settlement installment period     9 years      
PBGC Agreement [Member]            
Loss Contingencies [Line Items]            
Required pension contributions above minimum           $ 17,400,000
Pension contributions     $ 0 $ 0 $ 0  
Pension contributions above minimum remaining     $ 9,600,000      
[1] As adjusted due to the adoption of ASU 2016-18 "Statement of Cash Flows (Topic 230) Restricted Cash."