Fair-Value Measurements (Placement of Assets and Liabilities Within the Fair Value Hierarchy Levels) (Details) - Recurring Fair Value Measures - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Level 1 | Commodity Derivatives
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Fair Value	$ 619	$ 3,900
Liabilities Fair Value	(4,960)	(1,405)
Level 1 | Preferred Distribution Rate Reset Election
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities Fair Value	0	0
Level 2 | Swap
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Fair Value	666	1,274
Liabilities Fair Value	(288)	(125)
Level 2 | Preferred Distribution Rate Reset Election
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities Fair Value	0	0
Level 3 | Commodity Derivatives
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Fair Value	0	0
Liabilities Fair Value	0	0
Level 3 | Preferred Distribution Rate Reset Election
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities Fair Value	$ (50,157)	$ (40,840)