Segment Reporting - Reconciliation of Retail Gross Margin to Income Before Income Tax Expense (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Segment Reporting Information [Line Items]
Income before income tax expense	$ 44,243	$ 22,645	$ 17,959	$ 1,525
Interest and other income	(322)	47	(1,005)	(707)
Interest expense	2,174	2,762	6,392	7,323
Operating income	46,095	25,454	23,346	8,141
Depreciation and amortization	9,496	13,917	31,963	39,797
General and administrative	27,629	25,695	94,352	83,522
Less:
Net asset optimization (expense) revenues	(254)	348	2,242	3,798
Net, gain (loss) on non-trading derivative instruments	12,307	18,117	(42,690)	(1,371)
Net, Cash settlements on non-trading derivative instruments			32,593	(6,189)
Retail Gross Margin	58,182	45,795	156,483	134,939
Non-trading
Less:
Net, gain (loss) on non-trading derivative instruments	12,528	17,888	(42,741)	(2,223)
Net, Cash settlements on non-trading derivative instruments	$ 12,764	$ 1,035	$ 33,677	$ (5,054)