Inventory Financing Agreements - Supply and Offtake Agreements (Details)		3 Months Ended
	Dec. 05, 2018 USD ($) mbpd	Mar. 31, 2019 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2018 USD ($)	Jun. 01, 2018 USD ($)	Dec. 01, 2016 USD ($) installment	Jun. 01, 2015 USD ($)
Supply Commitment [Line Items]
Supply and exchange agreement expenses		$ 18,710,000	$ 8,377,000
Supply and Offtake Agreements
Supply Commitment [Line Items]
Agreement extension term		1 year
Deferred payment arrangement, aggregate marginal increase	$ 2,500,000
Barrels of crude per day provided (up to) | mbpd	150
Supply and exchange agreement expenses		$ 1,700,000	700,000
Amount of deferred payment arrangement	$ 165,000,000
Percentage of receivables and inventory for deferred payment arrangement	85.00%
Deferral arrangement fee							$ 1,300,000
Capacity of deferred payment arrangement		142,100,000	77,400,000
Outstanding amount of deferred payment arrangement		128,500,000	68,400,000
Number of fee agreement payments | installment						18
Fee agreement receivable		2,800,000		$ 2,500,000	$ 2,200,000	$ 14,600,000
Supply and Offtake Agreements | London Interbank Offered Rate (LIBOR)
Supply Commitment [Line Items]
Margin on LIBOR rate	3.50%
Deferred payment availability fee	0.75%
Shipping and Handling | Supply and Offtake Agreements
Supply Commitment [Line Items]
Handling fees		$ 5,400,000	$ 4,800,000