Investments - Investment Income (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Investment [Line Items]
Total Investment Income	$ 2,341	$ 2,868
White Cliffs Interest
Investment [Line Items]
Total Investment Income	1,048	831
Advantage Joint Venture
Investment [Line Items]
Total Investment Income	2,241	1,862
Delaware Crossing Joint Venture
Investment [Line Items]
Total Investment Income	(1,183)	0
EPIC Y-Grade
Investment [Line Items]
Total Investment Income	(30)	0
EPIC Crude
Investment [Line Items]
Total Investment Income	0	0
Other
Investment [Line Items]
Total Investment Income	$ 265	$ 175