Transactions with Related Parties (Narrative) (Details) - USD ($) $ in Thousands		4 Months Ended	12 Months Ended
		Dec. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Poseidon Oil Pipeline Company [Member]
Related Party Transaction [Line Items]
Due from related parties		$ 2,200	$ 2,200	$ 1,600
Management fees revenue			8,400	7,900	$ 3,900
Net sales from related party	[1]		12,357	10,844	4,536
Costs charged by related party, included in operating costs	[1]		986	1,007	464
ANSAC [Member]
Related Party Transaction [Line Items]
Due from related parties		74,490	74,490
Net sales from related party		124,500	124,536	0	0
Costs charged by related party, included in operating costs		$ 2,200	$ 2,242	$ 0	$ 0

[1] We own a 64% interest in Poseidon Oil Pipeline Company, LLC.