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Revenue Recognition (Narrative) (Details)
T in Millions
Jun. 30, 2019
T
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2019-07-01  
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]  
Remaining performance obligations, fixed price contracts (in tons) 38.9
Remaining performance obligations, variable price contracts (in tons) 4.9
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2020-01-01  
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]  
Remaining performance obligations, fixed price contracts (in tons) 64.3
Remaining performance obligations, variable price contracts (in tons) 5.7