Derivatives (Schedule of Effect on Operating Results) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Commodity derivatives - futures and call options:
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Recognized in Income	$ (6,765)	$ 6,432	$ (8,463)	$ 13,811
Preferred Distribution Rate Reset Election | Other expense
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Recognized in Income	(188)	0	(2,094)	0
Designated as Hedging Instrument | Commodity derivatives - futures and call options: | Onshore facilities and transportation product costs
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Recognized in Income	(1,421)	5,546	(2,787)	11,832
Not Designated As Hedging Instrument | Commodity derivatives - futures and call options: | Onshore facilities and transportation product costs, sodium minerals and sulfur services operating costs
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Recognized in Income	(5,344)	886	(5,676)	1,979
Not Designated As Hedging Instrument | Natural Gas Swap Liability | Sodium minerals and sulfur services operating costs
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Recognized in Income	$ 90	$ 0	$ (185)	$ 0