Commitments and Contingencies - Schedule of Delivery Commitments (Details)	6 Months Ended
	Jun. 30, 2018 Number T
Commitments and Contingencies Disclosure [Abstract]
2018 Q3-Q4 | T	2,475,000
Tons, 2019 | T	2,020,000
Tons, 2020 | T	1,146,000
Number of customers, 2018 Q3-Q4 | Number	18
Number of customers, 2019 | Number	8
Number of customers, 2020 | Number	5