Revenue Recognition - Narrative (Details) T in Millions	Jun. 30, 2018 T
Revenue from Contract with Customer [Abstract]
Remaining performance obligations, fixed price contracts (in tons)	70.8
Remaining performance obligations, variable price contracts (in tons)	6.4
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2018-07-01
Revenue from Contract with Customer [Abstract]
Remaining performance obligations, fixed price contracts (in tons)	35.0
Remaining performance obligations, variable price contracts (in tons)	4.9