Derivatives (Schedule of Effect on Operating Results) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Commodity derivatives - futures and call options
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Recognized in Income	$ (398)	$ (4,728)	$ (8,861)	$ 9,083
Preferred Distribution Rate Reset Election | Other income (expense)
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Recognized in Income	1,826	(2,276)	(268)	(2,276)
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	759	(3,399)	(2,028)	8,433
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	(1,157)	(1,329)	(6,833)	650
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	$ 229	$ 0	$ 44	$ 0