Employees - Narrative (Details) - USD ($)	6 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jun. 30, 2018
Defined Benefit Plan Disclosure [Line Items]
Liability for defined benefit plan					$ 15,300,000
Benefits paid under Union Agreement	$ 2,400,000	$ 9,100,000
Employer contribution	5,300,000	24,200,000
Employee contributions to the plan		0
Defined Benefit Plan, Expected Future Benefit Payment [Abstract]
Expected benefit payments, 2019		5,800,000
Expected benefit payments, 2020		6,000,000.0
Expected benefit payments, 2021		6,300,000
Expected benefit payments, 2022		6,700,000
Expected benefit payments, 2023		6,700,000
Total expected payments		$ 37,100,000
Employer match of 401(k) contributions		8.00%
401(k) expense		$ 9,600,000	$ 6,500,000	$ 3,800,000
Liability for other postretirement benefit plan	$ 3,900,000	$ 3,300,000	$ 3,900,000
Tyler Refinery
Defined Benefit Plan Disclosure [Line Items]
Percentage of employees covered by collective bargaining agreement		63.00%
Percentage of truck drivers covered by collective bargaining agreement		84.10%
El Dorado Refinery
Defined Benefit Plan Disclosure [Line Items]
Percentage of employees covered by collective bargaining agreement		61.20%
Big Spring Refinery
Defined Benefit Plan Disclosure [Line Items]
Percentage of employees covered by collective bargaining agreement		70.10%
Union Agreement
Defined Benefit Plan Disclosure [Line Items]
Expected total payment, under Union Agreement		$ 6,900,000
Benefits paid under Union Agreement		800,000
Defined Benefit Plan, Expected Future Benefit Payment [Abstract]
Expected benefit payments, 2019		2,000,000.0
Expected benefit payments, 2020		2,000,000
Expected benefit payments, 2021		2,000,000
Expected benefit payments, 2022		$ 100,000