Fair-Value Measurements (Placement of Assets and Liabilities Within the Fair Value Hierarchy Levels) (Details) - Recurring Fair Value Measures - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Level 1 | Commodity Derivatives
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Fair Value	$ 78	$ 1,754
Liabilities Fair Value	(872)	(5,459)
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 | Swaps
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Fair Value	0	616
Liabilities Fair Value	0	0
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	$ (85,154)	$ (52,372)