Transactions with Related Parties (Schedule of Transactions with Related Parties) (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Sandhill Group LLC
Related Party Transaction [Line Items]
Related party transaction, revenues	$ 690	$ 726	$ 1,233	$ 1,403
Equity method investment, ownership percentage	50.00%		50.00%
Poseidon Oil Pipeline Company
Related Party Transaction [Line Items]
Related party transaction, revenues	$ 3,039	3,044	$ 6,239	6,066
Related party transaction, costs and expenses	$ 250	249	$ 499	490
Equity method investment, ownership percentage	64.00%		64.00%
ANSAC
Related Party Transaction [Line Items]
Related party transaction, revenues	$ 93,938	0	$ 184,734	0
Related party transaction, costs and expenses	1,256	0	3,034	0
Chief Executive Officer
Related Party Transaction [Line Items]
Related party transaction, costs and expenses	$ 165	$ 165	$ 330	$ 330

The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 323
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (a)(1)
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+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 850
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=6457730&loc=d3e39549-107864

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Amount of revenue, fees and commissions earned from transactions between (a) a parent company and its subsidiaries; (b) subsidiaries of a common parent; (c) an entity and trusts for the benefit of employees, for example, but not limited to, pension and profit-sharing trusts that are managed by or under the trusteeship of the entity's management; (d) an entity and its principal, owners, management, or members of their immediate families; and (e) affiliates.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 946
-SubTopic 220
-Section S99
-Paragraph 1
-Subparagraph (SX 210.6-07.1(c))
-URI http://asc.fasb.org/extlink&oid=114873854&loc=SL114874292-224272

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