Commitments (Narrative) (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2018 USD ($) item
Commitments [Line Items]
Purchase commitment amount purchased	$ 27,078
Minimum [Member]
Commitments [Line Items]
Purchase commitment quantity | item	150,000
Purchase commitment	$ 11,104
Maximum [Member]
Commitments [Line Items]
Purchase commitment quantity | item	245,900
Purchase commitment	$ 51,028