Inventory Financing Agreements - Supply and Offtake Agreements (Details)		3 Months Ended		6 Months Ended
	Dec. 05, 2018 USD ($) mbpd	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 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		$ 20,278,000	$ 10,544,000	$ 38,988,000	$ 18,921,000
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		1,400,000	1,300,000	$ 3,100,000	2,000,000
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		142,000,000	77,400,000	142,000,000	77,400,000
Outstanding amount of deferred payment arrangement		126,900,000	68,400,000	126,900,000	68,400,000
Number of fee agreement payments | installment								18
Fee agreement receivable		1,000,000		1,000,000		$ 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		$ 7,700,000	$ 6,000,000	$ 13,000,000	$ 10,800,000