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, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Segment Reporting Information [Line Items]
Income before income tax expense	$ 27,747	$ 44,243	$ 72,190	$ 17,959
Interest and other income	(80)	(322)	(293)	(1,005)
Interest expense	1,487	2,174	4,233	6,392
Operating income	29,154	46,095	76,130	23,346
Depreciation and amortization	7,278	9,496	24,084	31,963
General and administrative	19,080	27,629	66,087	94,352
Less:
Net asset optimization (expense) revenues	(558)	(254)	(319)	2,242
Net, gain (loss) on non-trading derivative instruments	2,451	12,307	(14,015)	(42,690)
Net, Cash settlements on non-trading derivative instruments			32,682	32,593
Retail Gross Margin	47,031	58,182	147,486	156,483
Non-trading
Less:
Net, gain (loss) on non-trading derivative instruments	2,550	12,528	(14,019)	(42,741)
Net, Cash settlements on non-trading derivative instruments	$ 6,489	$ 12,764	$ 33,153	$ 33,677