Segment Reporting - Reconciliation (Details) - USD ($) $ in Thousands		3 Months Ended				6 Months Ended
		Jun. 30, 2018		Jun. 30, 2017		Jun. 30, 2018		Jun. 30, 2017
Segment Reporting Information [Line Items]
Income (loss) before income tax expense (benefit)		$ 27,178		$ 5,080		$ (21,120)		$ 18,617
Interest and other income		(553)		265		(754)		66
Interest expense		2,316		2,452		4,561		5,897
Operating income (loss)		28,941		7,797		(17,313)		24,580
Depreciation and amortization		12,861		9,656		25,880		18,926
General and administrative	[1]	27,780		19,346		57,827		43,839
Less
Net asset optimization revenues / (expenses)		763	[1]	(168)	[2]	3,450	[2]	(361)	[2]
Net, gain (loss) on non-trading derivative instruments		17,054		(9,677)		(19,488)		(31,473)
Net, Cash settlements on non-trading derivative instruments						(7,170)		11,828
Retail Gross Margin		43,425		43,149		89,144		107,749
Non-trading
Less
Net, gain (loss) on non-trading derivative instruments		16,601		(10,202)		(20,111)		(31,578)
Net, Cash settlements on non-trading derivative instruments		$ 8,793		$ 4,020		$ (6,089)		$ 11,535

[1] General and administrative expense includes general and administrative expense—affiliates of $1,600 and $6,100 for the three months ended June 30, 2018 and 2017, respectively, and $8,000 and $13,400 for the six months ended June 30, 2018 and 2017, respectively.

[2] Net asset optimization revenues (expenses) includes asset optimization revenues—affiliates of $340 and $0 for the three months ended June 30, 2018 and 2017, respectively, and asset optimization revenues—affiliates cost of revenues of $24 and $0 for the three months ended June 30, 2018 and 2017, respectively, and asset optimization revenues—affiliates of $988 and $0 for the six months ended June 30, 2018 and 2017, respectively, and asset optimization revenue—affiliates cost of revenues of $36 and $0 for the six months ended June 30, 2018 and 2017, respectively.