Segment Reporting - Reconciliation of Retail Gross Margin to Income Before Income Tax Expense (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Segment Reporting Information [Line Items]
Income before income tax expense	$ 11,993	$ 3,786
Interest and other income	(160)	(189)
Interest expense	1,553	2,223
Operating income	13,386	5,820
Depreciation and amortization	8,796	12,155
General and administrative	25,676	29,476
Less:
Net asset optimization revenues	321	2,552
Net, loss on non-trading derivative instruments	(24,587)	(19,541)
Net, Cash settlements on non-trading derivative instruments	16,293	7,106
Retail Gross Margin	55,461	56,577
Non-trading
Less:
Net, loss on non-trading derivative instruments	(24,533)	(19,803)
Net, Cash settlements on non-trading derivative instruments	$ 16,609	$ 8,125