Summary of Significant Accounting Policies - Turnaround Expenses (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Petroleum Segment
Planned Major Maintenance Activities [Line Items]
Turnaround costs	$ 4	$ 80	$ 31
Petroleum Segment \| Minimum
Planned Major Maintenance Activities [Line Items]
Frequency of planned major maintenance activities	4 years
Petroleum Segment \| Maximum
Planned Major Maintenance Activities [Line Items]
Frequency of planned major maintenance activities	5 years
Nitrogen Fertilizer Segment
Planned Major Maintenance Activities [Line Items]
Turnaround costs	$ 6	$ 3	$ 7
Nitrogen Fertilizer Segment \| Minimum
Planned Major Maintenance Activities [Line Items]
Frequency of planned major maintenance activities	2 years
Nitrogen Fertilizer Segment \| Maximum
Planned Major Maintenance Activities [Line Items]
Frequency of planned major maintenance activities	3 years