Transactions with Related Parties (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Poseidon Oil Pipeline Company
Related Party Transaction [Line Items]
Due from related parties	$ 1,900		$ 1,900		$ 2,200
Revenue from Related Parties	3,039	$ 3,044	6,239	$ 6,066
Related party transaction, costs and expenses	250	249	499	490
ANSAC
Related Party Transaction [Line Items]
Due from related parties	72,792		72,792		$ 74,490
Revenue from Related Parties	93,938	0	184,734	0
Related party transaction, costs and expenses	1,256	$ 0	3,034	$ 0
Management Service \| Poseidon Oil Pipeline Company
Related Party Transaction [Line Items]
Revenue from Related Parties	$ 2,200		$ 4,300

For an unclassified balance sheet, amounts due from related parties including affiliates, employees, joint ventures, officers and stockholders, immediate families thereof, and pension funds.

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