Commitments and Contingencies - Long-Term Fixed Supplier Commitments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Years ended, December 31:
2018	$ 9,936
2019	3,312
Total	13,248
Annual fixed fees incurred	$ 18,400	$ 23,000	$ 12,800