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Transactions with Related Parties (Schedule of Transactions with Related Parties) (Details) - Affiliated Entity - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Sep. 30, 2018
CEO
Related Party Transaction [Line Items]
Costs and expenses	$ 165	$ 165	$ 330	$ 330
Sandhill Group Llc
Related Party Transaction [Line Items]
Revenues	0	690	0	1,233
Equity method investment, ownership percentage					50.00%
ANSAC
Related Party Transaction [Line Items]
Revenues	81,784	93,938	172,463	184,734
Costs and expenses	1,279	1,256	2,336	3,034
Poseidon
Related Party Transaction [Line Items]
Revenues	3,236	3,039	6,401	6,239
Costs and expenses	$ 255	$ 250	$ 502	$ 499
Equity method investment, ownership percentage	64.00%		64.00%