Investments - Investment Income (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Investment [Line Items]
Total Investment (Loss) Income	$ (1,748)	$ 4,091	$ 593	$ 6,959
White Cliffs
Investment [Line Items]
Total Investment (Loss) Income	841	973	1,889	1,804
Advantage Joint Venture
Investment [Line Items]
Total Investment (Loss) Income	1,513	2,936	3,754	4,798
Delaware Crossing Joint Venture
Investment [Line Items]
Total Investment (Loss) Income	(355)	0	(1,538)	0
EPIC Y-Grade
Investment [Line Items]
Total Investment (Loss) Income	(1,024)	0	(1,054)	0
EPIC Crude
Investment [Line Items]
Total Investment (Loss) Income	(3,245)	0	(3,245)	0
Other
Investment [Line Items]
Total Investment (Loss) Income	$ 522	$ 182	$ 787	$ 357