Revenue Recognition - Narrative (Details) T in Millions	Mar. 31, 2018 T
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2018-04-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligations, fixed price contracts (in tons)	51.4
Remaining performance obligations, variable price contracts (in tons)	4.0
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: (nil)
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligations, fixed price contracts (in tons)	48.6
Remaining performance obligations, variable price contracts (in tons)	6.4