Segment Reporting - Reconciliation of Retail Gross Margin to Income Before Income Tax Expense (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Segment Reporting Information [Line Items]
Income before income tax expense	$ 32,450	$ (30,070)	$ 44,443	$ (26,284)
Interest and other income	(53)	(494)	(213)	(683)
Interest expense	1,193	1,995	2,746	4,218
Operating income	33,590	(28,569)	46,976	(22,749)
Depreciation and amortization	8,010	10,312	16,806	22,467
General and administrative	21,331	37,247	47,007	66,723
Less:
Net asset optimization (expense) revenues	(82)	(56)	239	2,496
Net, gain (loss) on non-trading derivative instruments	8,121	(35,456)	(16,466)	(54,997)
Net, Cash settlements on non-trading derivative instruments			26,256	19,891
Retail Gross Margin	44,994	41,724	100,455	98,301
Non-trading
Less:
Net, gain (loss) on non-trading derivative instruments	7,964	(35,466)	(16,569)	(55,269)
Net, Cash settlements on non-trading derivative instruments	$ 10,055	$ 12,788	$ 26,664	$ 20,913